424 Dr. Leeson on the Circular Polarizatio7i of Light 



which produces a deviation of 180°, f'lirnislies a very useful 

 index of their comparative circularly polarizing energy. 



If the depth be constant and two circularly polarizing liquids 

 be mixed together, or if the fluid or substance be dissolved or 

 mixed in some other fluid possessing no rotating energy, the 

 deviation will bear a direct relation to the proportion in which 

 the two are combined. For instance, since, according to my 

 own experiments, 10 inches of a solution of cane-sugar in 

 water containing half its weight of solid sugar produces a de- 

 viation of 104°, on mixing such solution with an equal bulk 

 of water, 10 inches of such diluted solution would produce 

 a rotation of only half that amount, or 52°, which coincides 

 with experiment; having, therefore, once obtained the rotating 

 power of a solution of given strength, we may, according to 

 Biot, calculate thence the strength of any other solution when 

 examined in a similar manner ; attention, however, must be 

 paid to the effects of temperature, any condensation which may 

 take place on mixture, specific gravity, &c. ; and I have not 

 found the result of experiments quite accord with those of 

 calculation, although the difference has been but slight, and 

 this is probably due in some degree to the circumstances 

 hereafter mentioned. 



The rules of proportion do not directly apply to the per- 

 centage of sugar by weight contained in solution, unless the 

 specific gravity and bulk be likewise taken into account. For 

 instance, the solution referred to was prepared as follows: — 



1000 grains dry sugar, 



1000 ... water, 

 and its specific gravity was 1'2371 ; consequently 1 gallon of 

 such solution would weigh 96,597 grains, and contain 48298*5 

 grains sugar. Now since 10 inches of such a solution produced 

 a rotation of 104°, on adding an equal bulk, that is, another 

 gallon of water to the original gallon, 10 inches would pro- 

 duce a rotation of 52°; but 1 gallon of water weighs 70,000 

 grains, which added to the weight of the syrup, 96,597 grains, 

 gives the weight of 2 gallons, 166,597 grains, supposing no 

 condensation to arise from the mixture; consequently 1 gal- 

 lon of the new solution would weigh 83298 "5 grains ; and since 

 it contains half the original quantity of sugar, or 24149*25 

 grains, the proportion by weight will be 24149*25 sugar to 

 59149*25 water, since 24149-25 + 59149*25 = 83298*5; so 

 that the proportion of sugar by weight to the water is less than 

 one-half, which would have been the case were the rotation in 

 direct proportion to the weight of sugar as compared with 

 that of the water in which it is dissolved, without reference 

 to the bulk occupied by the mixture. 



