420 M. Verdet on the Optical Properties developed in 



parent substances. In order, however, that no doubt might 

 remain on this point, I repeated on unequally concentrated 

 solutions of iron salts, the experiments which had been performed 

 on zinc and tin salts. For this purpose choice was made of two 

 protosalts of iron, — the protosulphate and the ])rotochloride, — 

 which may readily be obtained free from all traces of peroxide. 

 M. Deville furnished mc with a suflicient quantity of the sul- 

 phate, pKparcdwith the greatest care, from which two solutions 

 were made, the one containing 17'4 per cent., and the othei' 

 10'5 per cent, of anht/drous sidphate. The densities of these 

 solutions were respectively 1-1932 and 1*1135, and their mag- 

 neto-rotary powers 0-740 and 0-838 respectively. On applying 

 the same hypothesis and the same calculations to these solu- 

 tions, which were given above in the case of protochloride of tin 

 and chloride of zinc, two nearly equal values are obtained for 

 the molecular-rotary power of protosulphate of iron, viz. — 1"21 

 and ~l-35. These values, having the opposite sign to that of 

 water, and being sufficiently concordant, support the hypothesis. 

 Experiments on the protochloride of iron led to the same result. 

 The solution mentioned on page 419, made by dissolving piano- 

 forte wire in pure hydrochloi'ic acid, gave the number —0-91? as 

 the molecular power. A dilute solution, prepared by adding 

 pure water to the preceding one, and containing only 16 per 

 cent, of protochloride, gave the number — 0-82*. 



It is therefore proper to distinguish two kinds of action 

 exerted by transparent bodies subjected to magnetism upon 

 polarized light. I shall call the rotary power of water and of 

 most other non-magnetic transparent substances, positive ; that 

 of protosalts of iron and of bodies whose influence is in the same 

 direction, negative. These terms are preferable to the expres- 

 sions direct and inverse, which I employed in a previous publi- 



* The results of these experiments are worthy of consideration in an- 

 other point of view. It is seen that the oljserved i)ha2nomena were repre- 

 sented nimiericallj' under the supjiosition that tlie solutions of the sulphate 

 were formed of water and anhydrous sulphate, and hy ascribing to the 

 water and the salt actions contrary in direction and pro])ortional to the 

 densities which the two bodies had in the solution. If, on the contrary, 

 the solutions are supposed to consist of water and of crystallized sulphate, 

 containing 7 atoms of water, two values, viz. — 0'2 and — O'S, are found 

 for the molecular power, which are much more discordant than those got 

 on the previous sujii^osition. Similarly, on the supposition that the solu- 

 tions of the protochloride contain the salt crystallized with 4 atoms of 

 water, the numbers — 0'51 and — 0'13 are found for the molecular power 

 of the body: two values which are quite irrcconcileable. It hence appears 

 to follow with some probability, that in l)()tli cases tlie salt dissolved is not 

 the crystallized sul])hate or chloride, but the anhydrous sulphate and an- 

 liydrous chloride. I imagine that the same kind of experiment might be 

 applied to the solution of certain analogous chemical questions. 



