38 CHEMISTRY OF DISEASE. 



from the sound one, and although not absolutely destroyed 

 for the purposes of man, it is frequently very much im- 

 paired. 



(146.) In examining potatoes chemically, we must bear 

 in mind that different potatoes vary much in composition, 

 and even the same potatoe at different periods of time. 



(147.) The rough analysis of the potatoe is very easily 

 performed, when we desire merely to know the relative 

 constituents most useful to man. A given weight of the 

 tubers should be rasped to a fine powder, these raspings 

 should be placed in cold water and thoroughly washed, to 

 separate the starch from the fibre : the liquid should be 

 strained through a fine sieve several times, and allowed to 

 stand to deposit the starch. The fibre remaining in the 

 sieve should be placed in a piece of linen, and the water 

 pressed from it ; and, upon being dried in a water-bath, it 

 will give the full amount of starch. This may be placed 

 in a crucible over a clear fire, and thus the quantity of 

 ash may be learnt. After the starch has settled, the water 

 should be very carefully poured off and evaporated to dry- 

 ness at a moderate heat, when the quantity of albumen, 

 mucus, &c., will be indicated. The starch should then 

 have more water poured upon it, and be allowed to settle, 

 after which the water should be poured away from the 

 sediment ; this process should be repeated with fresh por- 

 tions of water until the supernatant liquid remains quite 

 colorless. The water may then be finally poured off, when 

 the starch must be very carefully dried in a dry room be- 

 fore the fire, and its weight ascertained. The amount ot 

 water would at last of all be inferred from the loss. 



(148.) Analyses of rotten potatoes are really of very lit- 

 tle use, because, in every state of rottenness, great differ- 



