510 
PROFESSOR HENRY A. HIERS: AX ENQUIRY INTO 
appears to be no sudden break in tlie curvature of the curve which expresses the 
relation lietween refractive index and constitution. 
They must not be regarded as more than a first attempt to estaldish this fact and 
to determine the curve for four substances ; in the case of the supersaturated 
solutions, the stirring was not alwa};s satisfactory, on account of the danger f)f making 
the solution crystallise ; and in the same series of observations the constitution was 
only deduced by comparison with another series. However, the very fair coincidence 
between tlie determinations made quite independently by the two methods gives 
some confidence in the experiments next to be described, in which a growing crystal 
itself is employed as the totally reflecting prism. 
(C.) The Refractive Index and Concentration of the SolutiGnin Contact with Growing 
Crijstcds of Alum, Sodium Chlorate, and Sodium Nitrate. 
As explained above, the growing crystal itself may in some instances be used as 
tlie totally reflecting prism, and it will then give the refractive index of the solution 
in contact with the crystal hj the formula on p. 495. Some difficulty will, of course, 
be experienced by reason of the vicinal faces ; in the first place, the angle of the 
prism will have to be measured during, or immediately after, each observation ; this 
can be done without withdrawing tlie crystal from the solution if the square trough 
be employed ; in the second place, and this is a more serious difficulty, the prism 
faces are no longer single plane surfaces, but sets of vicinal faces, and it will be 
necessary to select such as are very flat, or to make use of the growing crystal at the 
precise moment when two large vicinal faces can be employed as a prism and the rest 
are sufficiently small to be ignored. In spite of these difficulties, observations have 
been made upon the two cubic substances, 
alum and sodium chlorate, and upon sodium 
nitrate, which serve to determine approxi¬ 
mately the refractive index of the liquid, 
and therefore, by the help of the preceding- 
diagrams, its constitution. 
Since the light, on emerging from the 
ciystal, traverses a liquid of varying density, 
and since the crystal face through which it 
emerges is not parallel to the side AB of the 
trough nearest to the telescope T, the 
formula on p. 495 is not rigidly applicable 
(see fig. 18). 
If /X be the index of the crystal, the 
unknown index of the solution in contact 
with it, and gj the index of a saturated solution, Avhich is known from the preceding 
