Sunday 13 March 2011

Snail reproduction and mollusc birth control

One of my hobbies is snail-rearing. I started this when living in Dublin. In spring/summer 2007 it was so amazingly rainy (even for Irish people!) that thousands of snails could be seen in the gardens. It must have been really good news for birds.

I started to be interested in these animals, as I did not see many of them in dry, hot and full-of-concrete Madrid. When I left Ireland I brought three garden snails with me. And I continued my "collection" rearing snails from other countries such as Portugal, Andorra or Spain.

I saw them mating many times, but they appeared to be sterile, as they did not seem to be laying any eggs at all. Then I informed myself a bit, and I read that apparently this was because I was not putting any earth in the snail's box.

So I decided to make an experiment. There you go!

The idea: it is quite simple, the hypothesis to prove is

When earth is available, two snails will lay many fertile eggs after mating.

The material: a glass jar, two snails (apparently it does not matter the sex, as they are hermaphrodite) and some earth from the park down the street.

The method: The reason I used a jar was that in that moment I had seven snails in my snail box, and I did not want to have everyone's offspring, so I just chose two garden snails (one Irish and one Spanish, which happened to be called Yogurín and Tullido). The jar was small enough to keep them near to each other. To ensure ventilation I made small holes in the lid.

I left them in the "love jar" on 16th November 2009...

The measures: well, what can we mesure here? There are a few interesting things to measure, such as the amount of eggs, their viability and the time it took the little ones to hatch.

Happily, in less than 24 hours (!), on 17th November, at 1:20 AM, one of the lovers started laying eggs, here is the photo of that moment:

You can see about five eggs, and the head of the snail almost an inch into the earth. After some hours, the job was finished (next photo was taken at 9:55 PM). About sixty eggs buried and hidden from possible predators!

You can see them a bit better here:

Just in case you want to compare sizes, here they are next to a 50 euro cent coin:

As you can see, the egg cluster looks as big as the parent, it is incredible the fact that he/she was able to leave so many!

For logical reasons, I only kept four eggs. The other eggs were responsibly buried (and probably baby snails were born soon afterwards) in the very same spot where the Spanish parent was found, near the Alberche river, so no harm had been made to them, or to the environment (as there are already many similar snails mating and laying eggs over there anyway).

Here are the eggs I kept:

For some days I looked at the jar to see if the newborn snails were there. The eggs always looked exactly the same. Surprisingly, after eighteen days, on 5th December, they were there!

No shells appeared to be around (later I learned the first thing they eat is their own shell to get calcium). And the change was quite fast. A couple of days earlier they looked like white, small pellets, and then, suddenly... snails! Tiny, translucent but complete snails. For unknown reasons, only three out of four eggs were viable. The other one looked a bit elongated. But it is a good ratio anyway.

Just in case you are interested in sizes, here you have them on a one cent euro coin:

And here they are, in one of their first walks:

Conclusion: snail breeding is not that difficult! However, it seems I was a bit lucky as well. After this experience I tried to do the same with a couple of grove snails I brought from Portugal, and unhappily after many days they did not seem to give any results.

Just in case you wonder what happened to the snail family, the parents (after completing their normal life cycle) died at the age of three. One of the children disappeared (probably fled home, *sigh*). But the two remaining Irish-Spanish little ones grew into adulthood and have a happy snailly life at home. The photo below has been taken an hour ago.

Sunday 20 February 2011

How to compare heat capacities in your grandmother's kitchen

Performing simple sicentific measurements (such as getting the heat capacity of a given material) having proper scientific equipment is an easy, probably boring task.

However, things get funnier when our means are limited. For example, what about calculating and comparing the heat capacity of organic tissue and inorganic matter using only things you would find in any kitchen, such as your grandmother's kitchen? This means you cannot use the infrared thermometer your phisicist friend use to have in the drawer. And yes, you are right, the average grandmother does not use a microwave. Not at least nowadays.

So, first of all, we have to set an hypothesis to prove or disprove (otherwise, no science can be done, and we would wander from one thing to another... which might be interesting as well, but would not be really scientific).

The idea: following a small poll among a few non-scientific friends, I have found out that most of them would use a 20 kg hot stone instead of 20 kg of equally hot ham to put under the bed in a cold night. In other words,

A piece of granite would absorb more heat than a piece of ham the same weight.

so, according to this theory, if you had to store heat, you should use stones instead of food (provided that eating the food is cheating). Let us see if this is right.

The material: tap water, a freezer, a cooker, a small pot, three potatoes (sorry, no ham in my fridge, but these will do the trick), stones from the garden, a kitchen scale, a spoon (as stirring rod) and the timer of the cell phone (yes, I agree that the average grandmother would not have a cell phone, not at least in the kitchen, but you can always use your heartbeat... that is what Galileo did, and it worked pretty well for what he wanted to know about pendulums).

samples with 100 g of potatoes and 100 g of pabbles

The method: not having anything that resembles a thermometer, we will have to use as reference easy and a priori known temperatures. These could be the temperature of freezing water (0 ºC), the human body temperature (about 36 ºC) or the temperature of boiling water (100 ºC if you do not live too high; I live at 770 m, but this would just change this temperature by about 3 ºC, a negligible error if we compare with the error from other measures). To reduce the huge error we are going to have, we will use the widest range available: from freezing water to boiling water.

So, we first need to have boiling water in the pot. Then, we will drop in the 100 g of garden stones, which we have had for about five minutes in freezing water at 0 ºC, and calculate how long does it take for the water to boil again. Note that in order to make sure that the stones are at about 0 ºC, they must be in liquid water, but partially frozen; if we simply leave the stones in the freezer, they could easily reach temperatures of abuot -20 ºC.

We have to be as well careful when considering the boiling point: liquid water contains gases (air) dissolved in it, however, the amount of air dissolved is reduced with rising temperature (this is why salmons swim upriver, as cold water contains more oxygen for their offspring – you had one point in the chemistry exam if you knew this!), and this is the reason why a pot or a glass with hot water use to have little bubbles in its inner surface: they are the air that the water cannot keep any more. But the appearance of these bubbles must not to be confused with the posterior boiling point.

Then, we do the same for 100 g of raw potatoes and calculate the time the same way.

The heat capacity in J/kg·K will be:

(heat capacity in J/Kg•K) = (absorbed energy in J) / [(mass of pebbles/potatoes in kg) * 100 K]

You will say hey! how do I calculate the absorbed energy?. Well, that is why we are timing, as this can be calculated this way:

(energy absorbed by pebbles/potato in J) = (energy consumed by cooker in J) = (power of cooker in W) * (time in s)

Yet another small problem: we have to calculate the power of our cooker. So, this is when we start the next step...

The measures:

Calculation of the power of the cooker

100 mL samples of water during cooling process

Provided that the heat capacity of water is 4184 J/kg·K, I first used an indetermined amount of boiling water (to make sure the pot was already at about 100 ºC), dropped in 100 mL = 100 g of thawing/freezing (but not frozen) water, and calculated how long did it take for the water to boil again. I did this several times to have an average value:

WATER from time (s)to time (s)diffs/kg
100 g186.9275.788.7887
100 g275.7354.779.1791
100 g354.7434.679.9799
100 g434.6517.182.5825
100 g517.1614.297.1971

From these data we see that the average s/kg is 855 and the standard deviation is 75 s/kg. This way we can safely say that the power of our cooker is (490 ± 40) W (taking into account the propagation of error).

Calculation of the heat capacity of the potatoes

almost frozen potatoes about to be boiled, here the cell phone is used as stopwatch (the time the potatoes have been on the hand is negligible)

Three samples were used: two of 100 g and a smaller one of 28 g. They were cut in small pieces to make sure that, when water was boiling around the potato, the whole potato was at about 100 ºC. A spoon was used as stirring rod to guarantee the homogeneity of temperatures within the pot. This way, the following data were obtained:

POTATOfrom time (s)to time (s)diffs/kg
100 g0.091.091.0910
100 g134.7214.379.6796
28 g222.9247.724.8885

as the almost frozen potatoes enter the pot, the water stops boiling until the cooker gives enough heat to restore the boiling point

This gives us an average value for of 864 s/kg and a standard deviation of 60 s/kg, from which follows the specific heat of potatoes: (4200 ± 600) J/kg•K.

Calculation of the heat capacity of stones

We proceeded the same way, with the limitation that stones had not been broken into small parts.

STONESfrom time (s)to time (s)diffs/kg
100 g (several pebbles)319.6362.342.7427
137 g (conglomerate)384.0459.175.1548

More data could have been calculated, but as accuracy was not essential for this experiment, these were enough to get the average 488 s/kg and the standard deviation 86 s/kg, from which follows that the heat capacity of the stones were (2400 ± 600) J/kg•K.

The results: the error in the experiment is high, which is a consequence of the poor equipment available and the fact that we just garnered a few data. However, it is enough to see that organic tissue has about twice as much heat capacity compared to inorganic material.

Conclusion: potatoes (and presumably most of organic "living" matter) has a heat capacity very similar to the water: compare (4200 ± 600) J/kg•K to the 4184 J/kg•K of water. This is not surprising as it is well known that water is the main component of living matter.

Let us compare the (2400 ± 600) J/kg•K we have calculated for stones to the heat capacites of other inorganic substances, such as
  • iron: 450 J/kg•K
  • graphite: 710 J/kg•K
  • silicon: 703 J/kg•K
  • beryllium: 1820 J/kg•K
  • granito: 800 J/kg•K
  • sodium chloride: 854 J/kg•K
  • magnesium: 1020 J/kg•K

We can see that we have probably overestimated the heat capacity of our pebbles. Collecting more data and using bigger stones would provide more accurate information.

However, it is apparent the fact that for the same weight of pebbles and potatoes, the latter take quite longer to raise their temperature 100 ºC, which means they are storing more energy.

So, now we can safely refute the original hypothesis: it is indeed false that a piece of granite would absorb more heat than a piece of ham the same weight. Amusingly, and according to my fast poll, this seems to be counterintuitive.

I would be happy to read your comments about why should some people paradoxically think a stone is better than a piece of ham to keep oneself warm!