g6 REPORTS ON INVESTIGATIONS AND PROJECTS. 



With this apparatus, the mehing-points of tm, bismuth, cadmium, and lead 

 were determined, first at atmospheric pressure, and afterward with a gradual 

 increase of the pressure up to 2,000 atmospheres. The change of melting 

 temperature with pressure was found to be an accurately linear function of 

 the pressure. 



By substitution in the Clausius-Clapeyron equation of the data of Vicen- 

 tini and Omodei on the volume-change at the melting-point, and of Person 

 on the latent heat of fusion, dt/dp was calculated for each of the four 

 metals. The calculated values are in satisfactory agreement with those 

 observed. Incidentally, a convenient standard curve for the calibration of 

 copper constantan thermo-elements at temperatures from o to 425° has also 

 been computed and included in the paper, 



(13) The phenomenon of occlusion in precipitates of barium sudfate and its relation to 

 the exact determination of sulfate. John Johnston and L. H. Adams. J. Am. 

 Chem. Soc, 33, 829. 1911. 



The occlusion by barium sulphate of other sulphates is a general phe- 

 nomenon. The amount of this occlusion depends upon (a) the composition 

 of the original solution; (b) the fineness of the precipitate, which in turn is 

 conditioned by the degree of solubility of barium sulphate in the particular 

 medium, the rate of precipitation, and the time and manner of standing 

 between precipitation and filtration. The phenomenon is therefore in all 

 probability an absorption at the surface of the grains of the precipitate, since 

 it is affected by the factors just mentioned. 



On the basis of the knowledge gained in this way, attempts were made 

 to find a direct method for the determination of sulphate which should be 

 generally applicable, exact, and require only small and easily determined 

 corrections. The following procedure is suggested: To the solution (300 c.c. 

 for a precipitate to weigh 2 grams) add 50 c.c. of concentrated hydrochloric 

 acid, heat to boiling, and precipitate, stirring constantly, with a 10 per cent 

 solution of barium chloride. This should be added at such a rate that about 

 four minutes is required in running in the 22 c.c. necessary ; the rate is best 

 regulated by attaching a suitable capillary tip to the burette containing the 

 barium chloride solution. Evaporate the whole to dryness on the steam-bath 

 (this may be done immediately after precipitation), take up with hot water, 

 filter through paper, wash until the washings are free from chloride, ignite 

 very carefully (so as to obviate reduction), and heat to constant weight over 

 a Bunsen burner. The necessary correction is determined by a concurrent 

 calibration of the method; that is, by dissolving an equivalent weighed 

 amount of pure dry sodium (or potassium) sulphate in a medium such that 

 the resulting solution is as nearly as may be of the same composition as the 

 solution to be analyzed ; the sulphate in this comparison solution is then 

 determined precisely as above. The difference between the calculated amount 

 of barium sulphate and that actually found is the correction to be applied to 

 the weight of the precipitate obtained in the actual analysis. 



This procedure, as compared with that advocated by Allen and Johnston, 

 is easier and much more rapid ; it is, however, not so generally applicable, 

 but may be used whenever the composition of the solution containing the 

 sulphate to be determined is known approximately ; and, we believe, will 

 yield results, accurate to dz 0.05 per cent of the total sulphate present, in 

 most cases likely to occur in general analytical work. 



