CHEMISTRY. 



Ill 



dehydrated sulphate of lime, or, in other words, the 

 plaster of Paris formed by the calcination of tho 

 cement. The quantity of caustic lime which is pres- 

 ent in the cement keeps the plaster of Paris always 

 fresh, that is, dehydrated, until mixed with excess 

 of water employed at the moment of using it. This 

 will account for the fact that the cement does not lose 

 its quality by keeping, as the hydraulic cements do. 

 After the plaster of Paris is set, the caustic lime 

 goes on absorbing carbonic acid, and thus indurating 

 the mass in the ordinary manner of lime-mortars. 



The Phosphate Process for utilizing Sewage. 

 A second paper an this subject was read be- 

 fore the British Association during the year 

 by David Forbes, F. R. S., explaining the 

 operation of his process on the large scale. 

 To the disposal of sewage, by distributing it 

 over the land at once, there are several objec- 

 tions. The suspended and most offensive solid 

 matter cannot sink into the soil, but remains 

 upon the /surface, covering the place with its 

 fetid deposits, and polluting the atmosphere ; 

 moreover, sewage, though rich in ammonial 

 salt, is poor in phosphates, and therefore its 

 application to general farming is restricted. 

 It is claimed that, by the phosphatic process, 

 these difficulties are overcome. This con- 

 sists in treating the sewage with a solution of 

 the native phosphate of alumina dissolved in 

 sulphuric or hydrochloric acid, which arrests 

 decomposition and causes the precipitation of 

 the suspended matter. The liquid thus puri- 

 fied will now carry to the land a considerable 

 quantity of soluble phosphates in addition to 

 its other ingredients. 



The phosphate process, as applied to the 

 complete precipitation of the sewage, is in suc- 

 cessful operation at the Tottenham Sewage 

 Works, and goes a step farther than suggested 

 above, by precipitating all the fertilizing ma- 

 terials. The sewage is run into reservoirs 

 along with a suitable quantity of the phos- 

 phate solution, and immediately afterward 

 milk of lime is run in sufficient to neutralize 

 the acid of the phosphate solution. This pre- 

 cipitates the phosphates in solution together 

 with all the organic matter, and, after settling, 

 the supernatant water can be discharged into 

 the streams, without polluting them. The 

 purification requires from three to eight hours. 

 At the Tottenham Sewage "Works one ton of 

 such phosphate is employed for 500,000 gallons 

 of sewage. The phosphate is mixed, in a state 

 of powder, with 12 to 14 cwts. of sulphuric 

 acid. After standing a short time, it is dis- 

 solved in water and run into the sewage. 

 The milk of lime is added until the sewage 

 gives a faint alkaline reaction. An analysis of 

 some dried sewage deposit, thus produced from 

 London sewage, indicates 0.57 per cent, nitro- 

 gen, equal to 0.69 ammonia, and 28.52 per cent, 

 phosphoric acid, equal to 62.26 phosphate of 

 lime. The writer, however, does not think the 

 manure will pay the cost of the process, though 

 it will nearly do so, and is considerably more 

 advantageous and economical than the other 

 processes proposed. 



Changes of Color produced by Cold. Prof. 

 E. J. Houston, of Philadelphia, has published, 

 in the Journal of the Franklin Institute, a 

 statement of his investigations on this subject. 

 In his experiments on solids, the reduction of 

 temperature was obtained by the evaporation 

 of ether, bisulphide of carbon, or liquid sul- 

 phurous acid, the substances operated upon 

 being placed in vessels exposed to that cold- 

 producing action. The degrees of change in 

 temperature necessary to cause change in color 

 are not set down ; but the following are some 

 of the results with solids : 



Sulphide of Mercury. Changes from a bright red 

 to a brighter red. 



Bisulphide of Tin. Changes from a brownish 

 orange-yellow to a lighter brownish-yellow. 



Swsulphate of Mercury. Changes from a yellow 

 to a greenish-yellow. 



Iodide of Lead. Changes from an orange to a 

 lighter orange. 



Chromate of Lead. Changes from a yellowish- 

 orange to a yellowish-green. 



The effect of cold on the color of solutions 

 is thus stated : 



Sulphate of Copper. Solution of a pure blue ; 

 deepens on the application of cold. 



Ferrocyanide of Potassium. Saturated solution of 

 a nearly pure yellow ; becomes tinted slightly with 

 green. 



Chloride of Copper. Solution of a bluish-green; 

 becomes a more decided bluish-green. 



Sesquichloride of Iron. Solution, orange-yellow ; 

 becomes an orange-yellow in which the yellow is 

 more predominant than in the preceding. 



Sesquinitrate of Iron. Solution, orange-yellow, 

 like the chloride. 



Wishing to obtain a solution that could be exposed 

 to a much lower temperature without freezing, a so- 

 lution of the chloride of copper in ether was prepared. 

 The color was yellowish-green. ^ When exposed to a 

 low temperature by the evaporation of the bisulphide 

 -of carbon, the color changed very decidedly to a pure 

 green. 



The author adds : " It can hardly be urged, 

 with fairness, that all colored compounds 

 should be equally influenced by the action of 

 the less rapid heat-vibrations, for the differ- 

 ences presented by bodies, as regards their 

 transparency or opacity to light, or their dia- 

 thermancy or adiathermancy to heat, clearly 

 indicate a very great difference in their molec- 

 ular structure, which difference offers reasons 

 amply sufficient to explain why the colors of 

 some compounds should be more influenced 

 by heat than others. Again, there can be 

 little doubt that more extended observations 

 will increase the great number of compounds 

 already noticed. For instance, the well-known 

 change from red to yellow, presented by the 

 red iodide of mercury, dissuaded us at first 

 from submitting it to an experiment. On a 

 careful trial, however, it was found to illus- 

 trate the law, changing to a decidedly darker 

 red up to the temperature requisite to alter its 

 crystalline form. 



" The theory also receives further support 

 and confirmation from the following considera- 

 tions : 



" It is well known that when a yellow and a 



