27^ On the Definite Proportions in which the [April, 



Thenard and Gay-Lussac obtained 2"4 per cent, of the gum in 

 ashes, of which they kept an account in the exposition of the result 

 of their analysis of gum arable. These ashes consisted chiefly of 

 carbonMe of lime, the carbonic acid of which constituted nearly 

 one per cent of the gum. No attention was paid to this ; so that 

 what they considered as 100 gum, was in reality 101. Such an 

 error is too considerable to be neglected. 



Gum in its ordinary state contains water; but this water is a 

 hygroscopic substance, whose quantity varies according to the state 

 of the hygrometer. I have found that gum in powder, dried in a 

 vacuum at the temperature of 212, lost about 17 per cent. o£ 

 water. From this we may conclude, that gum ought to contain a 

 quantity of water, the oxygen in which is \ that in the gum. I 

 have not found that gum when dried slowly forms other combina- 

 tions with water. 



13. Potatoe Starch. — I took some potatoe starch, mixed it with 

 a little water, and poured it by small portions at a time into boiling 

 water, till I had obtained a solution of the requisite degree of con- 

 sistency. I passed it through a linen cloth, and then mixed it witii 

 a boiling solution of subnitrate of lead, which I added in excess, 

 and with which I digested the precipitate for a long time, that no 

 part of the starch might remain uncombined. I then washed the 

 precipitate by boiling it repeatedly in water, till that liquid ceased 

 to take up any more subnitrate of lead. I then subjected it to ^ 

 pressure, and dried it in a vacuum in the temperature of 212°. 

 This amylate, when analyzed by combustion, left 28 per cent, of 

 oxide of lead. Hence it is composed of 



Starch 72 100 



Oxide of lead 28 38-89 



100 



These 38"89 parts of oxide of lead contain 2-78 of oxygen. 



I endeavoured, likewise, to form an amylate containing more 

 base. I found that such a compound exists, though I was not pos- 

 sessed of any method of separating it in a state of purity. We 

 form it by mixing ammonia with a boiling solution of starch, and 

 precipitating the mixture by a boiling solution of subnitrate of lead. 

 But we cannot (unless by pure accident) find the point at which 

 no neutral amylate- js formed, or no subnitrate of the last degree. 

 A subamylate, which I endeavoured to obtain as near the point of 

 supersaturation as possible, without being mixed with insoluble 

 subnitrate, gav'e by combustion half its weight of oxide of lead ; 

 so that it seems to contain the starch combined with three times as 

 much base as in the neutral amylate. 



0-4 of starch, , dried in a vacuum, gave by combustion 0-2405 

 water, and G3<J5 carbonic acid, equivalent to 7'064 per cent, of 

 jtjydrogen, and-lS'-ISl of carbon. 



0-555 of amylate of lead, equivalent to 0-4 of starch, produced 



