Feb. 2, 1883.] 



KNOWLEDGE ♦ 



66 



THE CHEMISTRY OF COOKERY. 



By W. Mattieu Williams. 



n. — THK BOILING OF WATKU. 



AS this is one of the most rudimentary of the operations 

 of cookery, 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, 

 1st, 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 tii examine 

 it experimentally, but 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 tumbler-shaped vessels, so largely 

 used in chemical laboratories ; partially till it with ordinary 

 household water, and then place it over the flame of a spirit- 

 lamp, or Bunsen's, or other smokeless gas-burner. Care- 

 fully watch the result, and the following will be ob- 

 served: — First of all little buljbles will be formed, adhering 

 to the side* of the glass, but ultimately rising to the 

 surface, and there becoming dissipated by difi'usion in the 

 air. 



This is not boiling, as may be proved by trying the 

 temperature with the finger. What, then, is it 1 



It is the yielding back of the atmospheric gases which 

 the water has dissolved or condensed within itself. These 

 Dubbles have been collected and by analysis proved to 

 consist of oxygen, nitrogen, and carbonic 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 ditlerent samples of water. I need 

 not here go into the quantitative details of these propor- 

 tions, nor the reasons of their variation, though they are 

 very 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 iuuiiersed 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 not quite. Presently the buljbles rise com- 

 pletely to the surface and break there. Now the water is 

 boiling, and the thermometer stands at 212" Fahr. or 

 100° 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 removal of their last 



* In applying heat to glass vessels, thiclmess is a sonrce of 

 weakness or liability to fracture, un account of the unequal expan- 

 SJou of the two sides, due to inequality of temperature, which, of 

 course, increases with the thickness of the glass. Besides this, the 

 thickness increases the leverage of the breaking strain. 



traces being a very difficult, if not an impossible, physical 

 problem. 



After a moderate period of boiling, however, we may 

 practically regard the water as free from these gases. In 

 this condition I venture to call it cooked water. Our 

 experiment so far indicates one of the ditierences between 

 cooked and raw water. The cooked water has been 

 deprived of the atmospheric gases that the raw water 

 contained. By cooling some of the cooked water and 

 tasting it the dili'erence of flavour is very perceptible ; by 

 no means improved, though it is quite possible to acquire 

 a preference for this flat, tasteless liquid. 



If a fish be placed in such cooked water it swims for 

 awhile witli its mouth at the surface of tlie water, for just 

 there is a film that is reacquiring its cliarge of o.\ygen, iic, 

 by absorbing it from the air ; but this film is so thin and so 

 poorly charged, that after a short struggle the fish dies for 

 lack of oxygen in its blood, drowned as truly and conv- 

 pletely as a living, breathing animal when immersed in 

 any kind of water. 



Spring water and river water that have passed through or 

 over considerable distances in calcareous districts suflTer 

 another change in boiling. The origin and nature of this 

 change may be shown by another experiment as follows :— 

 Buy a pennyworth of lime-water 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 the tube or tobacco-pipe. Presently it 

 will become turbid. Continue the blowing, and the 

 turbidity will increase up to a certain degree of milkiness. 

 go on blowing with " commendable perseverance," and an 

 inversion of eflect will follow ; the turbidity diminishes, 

 and at last the water becomes clear again. 



The chemistry of this is simple enough. Prom the 

 lungs a mixture of nitrogen, oxygen, and carbonic acid is 

 exhaled. The carbonic acid combines with the soluble 

 lime and forms a carbonate of lime which 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, which 

 has been thrown down again by the action of boiling 

 in drawing off its solvent, the carbonic acid. This crust will 

 eft'ervesce if a little acid is added to it. 



In this manner our tea-kettles, engine boilers, <te., 

 become incrusted when fed with calcareous 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 lime, 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 run 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 micrococci, bacili, 

 bacteria, &.C., 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 



