1814.] On the Cause of Chemical Proportions. 25f 



sulphate ef rhodium, part of which dissolves in the acid, and another part remains 

 undissolved in the form of a black powder. AS the acid is evaporated, it deposites 

 more of this black powder ; and when the wliole acid is driven off, the persulphate 

 remains. This salt attracts moisture from the air, and becomes red. It dissolves 

 readily in water ; but when the water is evaporated, it does not form a black 

 powder, but a syrupy matter of an orange colour, which swells up in a greater 

 heat, and becomes spongy, like calcined alum. In that state it dissolves slowly in 

 water, just as happens to alum ; and though it appears at first scarcely soluble, 

 yet after two or three days we find it entirely dissolved. Caustic potash precipL- 

 lates from it a pale yellow mas-, which appears to he a triple subsulphate. 



I'ermuriale of Rliodium is already known by the experiments of Dr. Wollaston. 

 I shall only add that it is decomposed with more difficulty than the permuriate of 

 platinum ; for it is not altered at a temperature which destroys the platinum salt. 

 When the temperature is raised higher, it gives out muriatic acid and oxygen, and 

 leaves for residue muriate of rhodium. I mixed solutions of permuriate of rhodium 

 and common salt, but could not obtain the sodamuriate ef Wollaston. The liquid 

 retained its orange colour, and did not become red, even when evaporated to 

 dryness. The common salt appeared to crystallize without entering into combina« 

 tion with the permuriate of rhodium. When I heated the dryness to redness it 

 was decomposed, water dissolved the common salt, and left muriate of rhodium. 

 But when I kept the crystallized sodamuriate in a strong heat for a quarter of an 

 hour, it melted vi ithout undergoing decomposition. Its surface was covered with a 

 silvery metallic pellicle, but within it was unaltered, and dissolved in water with its 

 line red colour, leaving no other residue but the metallic pellicle. Hence it would 

 appear that somethin;;> else is necessary to form the sodamuriate of rhodium than a 

 mere mixture of peroxide of rhodium and common salt. 



Article III. 



Translation of a Letter from M. Mains to the Foreign Secretan/ 

 of the Royal Society. Communicated at the request of the 

 President.* 



SIR, Paris, June 1, 1811. 



I HAVE received your lettel- with the medal which the Royal 

 Society has done me the honour of voting for my Researches on 

 Light, inserted in the Memoirs of the Society of Arcueil. I am 

 very sensible of this distinction, and request you to make the 

 Society acquainted with my gratitude. I am fully aware of the 

 pains you have taken to make my experiments known ; and 

 since you take an interest in them, I shall embrace this opportunity 

 of making you acquainted with the further researches which I have 

 made on the subject. 



I had announced that when a ray of light was polarized, it might 

 traverse any number of diaphanous bodies witliout a single molecule 

 being reflected ; and 1 had added that the light, which would have 

 been reflected in the case of a polarized ray, had been transmitted, 

 and not absorbed or destroyed. The following is tlic diiect experi- 

 ment which you demand of me, and upon which I found tids pro- 



• The publication of thi« letter was deemed necessary in consequence of the 

 ubtervatiouk contulned in the iniruiluciiou to Dr. I5rewt>t<;r'« paper inserted in tb« 

 precedlug Number of the Annah uf P/iihiop/iy, 



Vol. III. N° IV. R 



