HOW THE ANIMAL LIVES 235 



more difficult to kill than the organism is when in an actively 

 growing condition. 



390. A precipitate, in chemistry, is a more or less solid 

 material, which is the result of chemical action, and which settles 

 to the bottom of the liquid in which it is formed. Thus, let the 

 pupil blow through a straw into a bottle of lime water. The 

 liquid will become cloudy, and after a time the sediment will 

 settle to the bottom. The pupil has added the carbon dioxid 

 (02) of his breath to the lime water, and carbonate of lime 

 (or limestone) has been formed. Compare 194a. 



392a. The action of the gastric juice may be familiarly seen 

 in the curdling of milk in the cheese factory by means of 

 rennet. A little mince-meat mixed with the scrapings of the 

 lining membrane of a pig's stomach, rendered slightly acid 

 by a drop or two of muriatic acid and kept near blood-heat 

 (96 F), will soon be completely dissolved, with the formation 

 of peptone. 



392fc. Rennet is the digestive principle derived from the 

 fourth stomach of ruminants (O, Fig. 87). This stomach is 

 taken from calves and dried ; and the stomach itself is then 

 spoken of as rennet. The stomach of adult animals could also 

 be used, if necessary. 



393a. The gastric apparatus of a chicken is shown in Fig. 

 88. The crop is at , the proventriculus at ft, and the gizzard 

 at c. 



396. An emulsion is that condition in which fatty or 

 oily materials are so intimately mixed with the liquid in which 

 they are placed that they act much as if they were in actual 

 solution, even passing through membranes. Most farmers are 

 now familiar with the kerosene emulsion, used as an insecti- 

 cide (296a). 



399a. Glycerin is a colorless liquid which is associated with 

 fats or fat-acids, and which may be derived from them. Its 

 composition is C3H5(OH)3. It is often made from the fats by 

 artificial means, and is used in medicine and the arts. Also 

 spelled glycerine. 



402(7. Two villi are shown in Fig. 89. The singular form of 

 the word ia v ill us. 



