OF ARTS AND SCIENCES. 15 



compactly it was easily washed in the same flask in which the solu- 

 tion was made and the precipitate formed. The precipitate usuiilly 

 occupied about oue-ti-iith of the voliune of the flask, and after drawing 

 off the filtrate our rule was to fill the flask five times with boiling 

 water. "We were sometimes troubled by the breaking up of the preci- 

 pitate and the washing of it through the filter, under such prolonged 

 treatment. As is well known, this can easily be prevented by acidifying 

 the wash water, however slightly, with hydrochloric acid, or by an excess 

 of H.^S ; but as the last was liable to oxidation, and the first still more 

 objectionable, for reasons which will soon appear, we employed for the 

 same purpose a solution of carbonic dioxide, and this, as we found, 

 effectually prevented tlie disintegration. If by not settling promptly 

 the preci|)itate showed any signs of breaking up, we j^assed a current 

 of CO2 through the hot water before it was used in the last one or 

 two washings. 



In all the later determinations we collected and dried the precipitate in 

 a large platinum crucil)le by the method of reverse filtering illustrated 

 and described at length on page 124 of the previous volume of these 

 " Proceedings." But, in the two determinations made by the method 

 just described, the precipitate was collected and dried on the porous 

 earthenware filtering cones described by Prof. C. E. Munroe.* The 

 cones with the precipitate were first dried at 100°, and then heated to 

 220°C. in an atmosphere of carbonic dioxide. Between 210° and 220°, 

 depending somewhat on its condition, the red sulphide of antimony 

 changes suddenly into the gray modification, and in this last condition 

 the product was weighed. In each of these two determinations, on 

 dissolving the gray sulphide in boiling hydrochloric acid, out of con- 

 tact with the air, a considerable .quantity of black residue was obtained. 

 This contained only a very small amount of free sulphur, but consisted 

 chiefly t of carbon, resulting from the charring of the small quantity of 

 tartaric acid, which, as we find, sulphide of antimony invariably carries 

 down with it when precipitated under the conditions here described, 

 and which cannot be removed by any amount of subsequent washing. 

 This charring takes place at the moment when the red is converted 

 into the gray sulphide, and is undoubtedly caused by the heat evolved 



* American Journal of Science, May, 1871. 



t In our preliminary experiments, traces of both lead and copper appeared 

 in this residue, which we traced at once to the tartaric acid " purissimum" used, 

 and we found it impossible to procure from chemical dealers tartaric acid free 

 from these impurities. We were obliged, therefore, to purify ourselves all tlie 

 acid we employed. 



