August 17, 1906.] 



SCIENCE. 



201 



concentration at Avhich the solubility- 

 changes. 



Another method was used in the identi- 

 fication of the insoluble phases formed 

 when thallium sulphide and stannic sul- 

 phide are precipitated together. Since the 

 phases have different colors the appearance 

 or disappearance of a phase can be readily 

 observed under the microscope. From pure 

 TI2S up to a concentration corresponding 

 to Tl^SnS^ two phases can be distinguished, 

 the black Tl^S, and the red Tl^SnS^; from 

 this point up to 76 per cent. SnSg two 

 phases are present, the red Tl^SnS^ and a 

 reddish yellow transparent phase, while be- 

 yond 76 per cent. SnSa all concentrations 

 are homogeneous, showing a solid solution. 



The Equilibrium 'between Ammonia and 

 Hydrogen Sulphide: J. P. Magnusson. 

 The reaction 



NH3 + H,S ^ NH,SH 



was studied at 20° over a range of 95 cm. 

 partial pressure. The mass-law formula 



i»NH3 X P^iS = const. 



describes this equilibrium over the pressure 

 range studied if the pressure of the undis- 

 sociated NH^SH is neglected and if correc- 

 tions are made for the deviations of the 

 gases from Boyle's law. For hydrogen 

 sulphide this deviation was found to be 

 within the limit of experimental error over 

 the pressure range studied, but for am- 

 monia the deviation was considerable at 

 the higher pressures. This was shown to 

 be due to the adsorption of the gas on the 

 glass surface of the measuring tube and on 

 the NH4SH crystals. The adsorption of 

 gases on the walls of the containing vessel 

 has an important bearing on our concep- 

 tion of the so-called imperfect gases. 



The Precipitation of ~ Lead Chromate: Ed- 

 ward E. Free. 



Precipitations w^ere made from equiva- 

 lent solutions of Pb(N03)2 and K2Cr04 un- 



der as nearly as possible equal conditions. 

 The rapidity of precipitation is greater 

 from hot solutions and more concentrated 

 ones. The crystals are large when the 

 solutions are hot and dilute. The presence 

 of glue greatly retards precipitation. The 

 color is apparently the same under all 

 conditions of precipitation, provided the 

 solutions are neutral. Alkali will impart 

 an orange tinge due to basic chromate. 

 Minor accidental variations in the manner 

 of precipitation have a great influence. 



Solubility of the Phosphates of Mag- 

 nesium: F. K. Cameron and J. M. Bell. 

 The behavior of calcium phosphates in 

 contact with water and with phosphoric 

 acid solutions has already been studied in 

 this laboratory by Cameron, Seidell and 

 Bell. It was found that at 25° only two 

 calcium phosphates exist in equilibrium 

 with aqueous solutions. At the higher 

 concentrations monocalcium phosphate is 

 the stable solid phase, and at intermediate 

 concentrations dicalcium phosphate is the 

 solid phase. At very low concentrations 

 there is at least one series of solid solutions. 

 The behavior of magnesium phosphates in 

 contact with water and with phosphoric 

 acid solutions has been studied in a similar 

 way at 25° C. Here there are but two 

 magnesium phosphates, the monomag- 

 nesium phosphate, which exists in contact 

 with solutions containing above 700 grams 

 of P2O5 per liter; below that concentration 

 the stable solid is dimagnesium phosphate. 

 The solution in contact with the above 

 phosphates of calcium contains about 320 

 grams of P2O5 per liter, while that in con- 

 tact with the phosphates of magnesium 

 contains over 700 grams of P2O5 per liter. 



The System Water-Gypsum-Lime: F. K. 



Cameron and J. M. Bell. 



The mutual solubility of gypsum in lime 

 solutions and of calcium hydroxide in 

 gypsum solutions has been determined at 



