THE CHEMISTRY OF COOKERY. II.—The Boiling Of Water.

Waikato Times, Volume XXI, Issue 1729, 4 August 1883, Page 6

THE CHEMISTRY OF COOKERY.

ll.— The Boiling Of Water.

DT W. JIATTIEU WILLIAMS.

As this is one of the most rudimentary of the operations of oookery, and the most frequently performed, it naturally takes a first place in treating the subject. Water is boiled in the kitchen for two distinct purposes, Ist, for the cooking of itself ; 2nd, for the cooking of other things. A dissertation on the difference between raw water and cooked water may appear pedantic, but, as I shall presently show, it is considerable, very practical, and important. The best way to study any physical subject is to examine it experimentally, bat this is not always possible with every-'day means. In'this case, however, there is no difficulty. Take a thin* glass vessel, such as a flask, or better, one of the " beakers," or thin tum-bler-shaped vessels, so largely used in chemical laboratories ; partially fill it with ordinary household water, and then place it over the flame of a spirit-lamp, or Bunsen's, or other smokeless gas-burner. Carefully watch the result, and the following will be observed : — First of all little bubbles will be formed, adhering to the sides of the glass, but ultimately rising to the surface, and there becoming dissipated by diffusion in the air, ♦ This is not boiling, as may be proved by trying the temperature with the finger. What, then, is it ? It is the yielding back of the atmospherio gases which the water has dissolved or condensed within itself. These bubbles have been collected and by analysis proved to consist of oxygen, nitrogen, and oarbonic acid, obtained from the air ; but in the water they exist by no means in the same proportions as originally in the air, nor in constant proportions in different samples of water. , I need not here go into the quantitative details of these proportions, nor the reasons of their variation, though they are yery interesting subjects. Proceeding with our investigation, we shall find that the bubbles continue to form and rise until the water becomes too hot for the finger to bear immersion. At about this stage something else begins to occur. ' Much larger bubbles, or rather blisters, are now formed on the bottom of the vessel, immediately over the flame, and they continually collapse into apparent nothingness. Even at this stage a thermometer immersed in the water will show that the boiling point is not reached. As the temperature rises, these blisters rise higher and higher, become more and more nearly spherical, finally quite so, then detach themselves and rise towards the surface ; but .the first that make this venture perish in the attempt— they gradually collapse as they rise, and vanish before reaching the surface. The thermometer now shows that the boiling-point is nearly reached, but noj; quite. Presently the bubbles rise completely to the surface and break there. Now the water is boiling, and the thermometer stands at 212 deg. Fahr. or 100 deg. Cent. With the aid of suitable apparatus it can be shown that the' atmospheric 'gases above named continue to be given off along with the steam for ,a considerable time after the boiling has • commenced ; the complete renqpval of their last traces being a very difficult, if not an impossible, physical problem. After a moderate period of boiling, however, we may practically regard .the. wateif, as free from these gases. In this: condition' I * In applying heat to glasc vessels, thickness, is a source of weakness' or liability to fracture, on.account of the unequal exp&mion of ihe two /sides*,, due to inequality of temperature, wnichj of co'urie, increases 'with the thickness of theglftssl Besides this; the' thickness increases' 'the leverage 1 of the' breaking strain. '„ " » *■ \ • y * '

venture to call it sooked water. Ou^ experiment so far indicates one of the differences between cookedyand raw /water,- TKe cogked wate|.has been , deprived •of the atmospheric gases that the' raw wiftter contained. By cooling 'some of the cooked water and tasting it the.differenceof flavour id: very perceptible ; by^no means improved, though it is quite possible 'to acquire t, preference for this flat, tasteless liquid. ' * If a fish be placed in subh cooked water it swims for awhile with its mouth at the surface of the water, for just there, is a film that is reaoq[qiring its charge of oxygen, <fee, by absorbing it from the air ; but this film is so thin and so poorly charged, that after a short struggle the fish die 3 for lack of oxygen in its blood, drowned as truly and completely as &* living, breathing animal when immersed in any land of water. Spring water and river water that have* passed through or over considerable distances in oalcareous distriots suffer another change in boiling. The origin and nature of this ohange may be shown by another experiment as follows :— Buy 'a pennyworth of limewater from a druggist, and procure a small glass tube of about quill size, or the stem of a fresh tobacco pipe may be used. Half fill a small wine-glass with the lime-water, and blow through it by means of tb^e tube or tobacco pipe. Presently it will become turbid. Continue the blowing, and the .turbidity will increase up to a certain degree of milkineßS. Go on blowing with " commendable perseverance,".and an inversion of effect will follow ; the turbidity diminishes, and at last the water becomes clear again. The chemistry of this is|imple enough. From the lungs a mixture of nitrogen, oxygen, and oarbonio acid is exhaled. The carbonic acid combines with the soluble lime and forms a carbonate of lime whioh is insoluble in mere water. But this carbonate of lime is to a certain extent soluble in water saturated with carbonic acid, and such saturation is effected by the continuation of blowing. . Now take some of the lime-water that has been thus treated, place it in a clean glass flask, and boil it. After a short time the flask will be found incrusted with a thin film of something. This is the carbonate of lime, whioh has been thrown down again by the action of boiling in drawing off its solvent, the carbonic acid. The crust will effervesce if a little acid is added to it. In this manner our tea-kettles, engine boilers, &c, become inorusted when fed with oaloareous waters, and most waters, are calcareous ; those supplied to London, which is surrounded by .chalk, are largely so. Thus the boiling or cooking of such water effects a removal of its mineral impurities more or less completely. Other waters contain such mineral matter as salts of sodium and potassium. These are not removable by mere boiling. Usually we have no very strong motive for removing either these or the dissolved carbonate of limes, or the atmospheric gases from water, but' there is another class of impurities of serious importance. These are, the organic matters dissolved in all water that has ran over land covered with vegetable growth, or, more especially, which has received contributions from sewers or any other form of house drainage. Such water supplies ' nutriment to those microscopic abominations, the ndcrocoeci, bacili, fyactcria, <£<;., which are now shown to be connected with blood poisoning — possibly do the whole of the poisoning business. These little pests are harmless, and probably nutritious, when cooked, but in their raw and wriggling state are horribly prolific in the blood of people who are in certain stales of what is called " receptivity." They (the bacteria, &o.) appear to be poisoned or somehow killed off by the digestive secretions of the blood of some people, and nourished luxuriantly in the blood of others. As nobody can be quite sure to which class he belongs, or may presently belong or whether the water supplied to his household is free from blood poisoning organisms, cooked water is a safer beverage than raw water. The requirement for this simple operation of cooking increases with the density of our population, whioh on reaching a certain degree renders the pollution of all water obtained from the ordinary sources almost inevitable. Eeflecting on this subject, I have been struck with a curious fact that has hitherto escaped notice, viz., that in the country whioh over all others combines a very large population with a .very smaU. allowance of cleanliness, the ordinary drink of the people is boiled water flavoured by an infusion of leaves. These .people, the Chinese, seem, in fact, to have been the inventors of boiled water beverages. Judging from travelers' accounts of the state of the rivers, rivulets, and general drainage and irrigation arrangements of China, its population could scarcely have reached its present density if Chinamen were drinkers of raw instead of cooked water.