OF ARTS AND SCIENCES. 257 



salt made by precipitating with the diplumbic urate a small amount 

 of plumbic hydrate. We have made no experiments to show how 

 small this quantity of plumbic hydrate can be made with safety, as it 

 seemed a matter of little importance. In preparing the lead salt we 

 dissolved uric acid in a potassic hydrate solution of known strength, 

 using twenty per cent more potassic hydrate than was theoretically 

 required to form the dipotassic urate, and poured this alkaline solution 

 into a boiling dilute solution of plumbic nitrate. After washing the 

 bulky precipitate with hot water we usually washed with alcohol, and 

 finally with a little ether; partly to facilitate the drying, and partly 

 because the lead salt was in this way obtained in a much more com- 

 pact and convenient form. After drying at 100° the salt was well 

 powdered and dried at 1 60°. 



In order to insure complete decomposition of the lead salt, which 

 evidently is essential to prevent the formation of monomethyluric 

 acid, we have found it necessary to take a slight excess of methyl 

 iodide, to dilute it with an equal weight of ether, and, after mixing 

 thoroughly with the lead salt, to heat for twenty hours at 1G">°. We 

 also have found it advantageous to allow the tube to cool after heat- 

 ing for twelve to fifteen hours, and to mix the contents thoroughly by 

 shaking before heating further. 



After distilling off the ether and the slight excess of methyl iodide, 

 the solid product of the reaction is extracted with boiling water, and 

 the lead precipitated by hydric sulphide. The solution filtered boil- 

 ing hot deposits on cooling crystals of dimethyluric acid which may 

 be purified by recrystallization from hot water. In this way we have 

 obtained about fifty per cent of the theoretical yield of well crystal- 

 lized product apparently pure. Since monomethyl and dimethyluric 

 acids differ in the amount of carbon they contain only by 3.29 per cent, 

 it is evident that the absolute purity of our product could not be 

 determined by analysis. 



Dimethyluric acid crystallizes usually in slender oblique prisms, 

 often concentrically grouped, which contain a molecule of crystal 

 water. From saturated solutions where crystallization takes place at 

 a high temperature we have not unfrequently obtained small, compact, 

 pointed prisms, which contain less water and very possibly are an- 

 hydrous. The behavior of both of these forms to polarized light shows 

 that they belong to one of the oblique systems. The acid begins to 

 turn brown when heated to about 340° ; at higher temperatures it 

 melts, with decomposition and partial sublimation. It is quite sol- 

 uble in boiling water, sparingly soluble in cold water, still less so in 

 vol. xv. (x. s. vn.) 17 



