30 PROCEEDINGS OF THE AMERICAN ACADEMY 



Transactions of the Medical Society of the State of New York.* In 

 order to purify the acid, it was repeatedly redistilled with a small 

 amount of a concentrated solution of bromide of potassium, rejecting 

 each time the distillate until the boiling point rose to 128°; when, as 

 is well known, an acid containing about 47 per cent of HBr distils 

 unchanged. The acid thus obtained was as colorless as water. 



Meanwhile, in order to test the purity of the hydrobromic acid and 

 also as a basis for the rest of our investigation, we jjrepared a quantity 

 of pure silver by the method already fully described in a previous 

 part of this volume (page 17) ; and with the pure silver thus pre- 

 pared the following two sets of determinations were made. 



For tlie first set, weighed amounts of silver were dissolved in very 

 carefully purified nitric acid, using only a very slight excess of this sol- 

 vent in any case. "We prepared for the purpose a dilute acid by mixing 

 one part of acid, having Sp. Gr. 1.355, with four parts of water and 

 of this weak acid 5. 3 c. m.^ were required for each gramme of silver. 

 We were thus able to estimate the amount necessary for each analysis, 

 and we used generally one half a cubic centimetre in excess. 



The silver having been perfectly dissolved, and the solution diluted 

 with water to from two hundred to five hundred cubic centimetres, 

 according to the amount of silver used, we gradually and cautiously 

 precij^itated bromide of silver by adding pure hydrobromic acid, pre- 

 pared as just described, but greatly diluted until the acid was very 

 slightly in excess. For the method of washing and collecting this 



* " The formula and process for making an acid of this strengtli are as 

 follows : — 



Take of Potassium Bromide Six parts. 



Sulphuric Acid, Sp. Gr. at 15°.6 C. I ^ 



'■ ' ' > . . . Seven parts. 



1.838, at 25 ° C . 1.828 ) 



"Water Nine parts. 



"Add to the sulphuric acid one part of the water and cool the mixture. 

 Then dissolve the potassium bromide in six parts of the water by means of heat, 

 supplying the loss of water by evaporation during the heating. Pour tlie diluted 

 sulphuric acid slowly into the hot solution witli constant stirring, and set the 

 ritixture aside for twenty-four hours, that the sulphate of potassium may cr3's- 

 tallize. Pour off the liquid into a retort, break up the crystalline mass, transfer 

 it to a funnel, and, having drained the crj'stals, drop slowly upon them two parts 

 of the water so as to displace and wash out the acid liquid. Add the liquid, thus 

 drained ofE and washed out, to that in the retort, and distil the whole nearly to 

 dryness, or until nothing further distils oflP by moderate heating. The distillate 

 will weigh about ten parts and should contain about thirty-seven per cent of 

 hydrobromic acid." 



