MINEKAL SALTS 55 



When a solution containing grape sugar is treated with a 

 few drops of this solution, a substance is precipitated, 

 which, upon heating, gives up part of its oxygen to the 

 sugar, and turns a reddish brown. If no sugar is present 

 this loss of oxygen does not occur and there is no con- 

 sequent colored precipitate. This reaction furnishes a 

 simple and delicate means of detecting the presence of 

 grape sugar in a compound. 



Fats. — Under this name are included not only what is 

 ordinarily understood by the term fat, but also animal and 

 vegetable oils of all kinds. Like the carbohydrates, fats 

 are composed of carbon, hydrogen, and oxygen, but in differ- 

 ent proportions. They are all lighter than water and in- 

 soluble in it. All are readily made liquid by a moderate 

 amount of heat, while some are liquid at ordinary tempera- 

 tures. Thvis body fat melts at a temperature of 98° 

 Fahrenheit and is liquid in the living body. (See Ex. XVI. 

 a.) Fats are soluble in ethers and benzene and when 

 heated with alkalies form soap. Their presence may be 

 detected in one of two ways. (See Ex. XVI., b, c.) 



(a) Rub on paper the substance to be tested or place 

 it on the paper and heat gently. If fat is present a grease 

 spot will result, which gives the paper a translucent appear- 

 ance and is readily detected. 



(b) Soak the substance in ether (benzene will do as 

 well). The ether will dissolve the fat if it is present. Then 

 filter off the solution and allow the ether to evaporate. 

 The fat does not evaporate and will remain. This second 

 test enables us to extract and measure the exact quantity 

 of fat present as well as to determine its presence. 



Mineral salts. — Many forms of salts occur dissolved in 

 water and are present in aU living matter, while other insol- 



