24 FIRESIDE SCIENCE. 



which indicates the presence of an alkali. The 

 alkali is soda in a free condition, and its presence is 

 of the highest consequence, for without it the liquid 

 would be insoluble. A portion of the white of 

 egg when diluted with water, and a few drops of 

 vineo-ar or acetic acid added to it, undergoes a 

 rapid change. The liquid becomes cloudy and 

 flocculent, and small bits of shreddy matter fall to 

 the bottom of the vessel. This is pure albumen, 

 made so by removing the soda held in combination 

 by the use of the acid. A pinch of soda added to 

 the solid precipitate redissolves it, and it is again 

 liquid. There is another way by which the albu- 

 men is rendered solid, and that is by the applica- 

 tion of heat. Eggs placed in boiling-hot water 

 pass from the soluble to the insoluble state quite 

 rapidly, or in other words, the albumen both of the 

 white and the yolk becomes " coagulated." No 

 contrast can be greater than that between a boiled 

 and unboiled egg. Not only is it changed phys- 

 ically, but thjre is a change in chemical properties, 

 and yet no chemist can tell in what the change 

 consists. It is true that water extracts a little 

 alkali, and a trace of sulphide of sodium, but the 

 abstraction of these bodies is hardly sufficient to 

 account for the change in question. 



The hardening of the albumen of egg by heat 

 constitutes the cooking process, and this deserves a 



