CHEMICAL SCIENCE. 193 



though a little earlier in the former. The action was found more 

 rapid and more complete in dilute than in concentrated solutions. 

 The latter, after having ceased to deposit, give a further amount of 

 precipitate if diluted. There is a considerable effervescence in the 

 liquid, caused by the escape of oxygen gas. The precipitate is at first 

 of a greenish yellow, but becomes a yellowish brown when dry, and 

 shrinks greatly in bulk. Solutions of sulphate of copper and of 

 bichromate, exposed to the sun, and then mixed in the dark, give the 

 same precipitate ; but if prepared in the dark, no perceptible precipitate 

 is obtained, even on long standing. If the mixture is boiled, however, 

 a precipitate appears. If the precipitate formed by the action of the 

 sun is removed and the liquid boiled, a precipitate appears of a redder 

 shade, probably identical with Bensch's chroniate of copper. I have 

 not as yet obtained a quantity sufficient for analysis, either of this or 

 of the former precipitate. Whether the electric and hygrometric 

 conditions of the atmosphere exert any influence iipon actinic decom- 

 positions, I have not been able to determine with certainty. 



OX THE APPLICATION OF CERTAIN OPTICAL PHENOMENA TO 



CHEMISTRY. 



PROF. STOKES at the British Association stated that he had found 

 in experimenting on prismatic fringes, and the dark lines of the spec- 

 trum, that by the interposition of small portions of chemical substances 

 in solution, and in other cases, as by the beads formed by the blow- 

 pipe, optical means would discover the presence of many bodies by 

 their power upon light. . Arguing upon the advantage of this auxiliary 

 power to the chemist, he pointed out the facility with which trials 

 could be made. The salts of per-oxide of uranium, for instance, has 

 a property of showing dark lines in a certain portion of the spectrum, 

 and on one occasion he discovered on a blowpipe bead the lines that 

 were usually associated with the presence of uranium ; in that instance 

 he had no reason to expect it could be present, and upon careful 

 attention he found that he had used a platina wire that had been 

 employed with experiments where uranium had been present ; and a 

 minute quantity must have remained attached to the wire, and thus 

 become evident. He took a single case of difficulty and where doubt 

 still remained, to call the attention of chemists to the value of optical 

 research where ordinary tests did not avail. He found that manga- 

 nesic acid in solution had a certain power over light, giving dark 

 bands. There was a class of crimson solutions of manganese which 

 some chemists supposed the per-oxide in solution, and others a differ- 

 ent oxide ; but Mr. Pearsall, had shown the probability that rnanga- 

 nesic acid was present : this was a fair case, and accordingly by optical 

 means he decided that manganesic acid was not present. He had 

 considered the effects of acids and bases, but sulphuric acid and potassa 

 made no difference if added to manganesic acid. He considered this 

 a fair case, when no test had been devised to settle the point. Prof. 



