282 J. H. Long—Indices of Refraction of Compound Ethers. 
From the above table it is seen, as has already been shown 
by Landolt and others, that the increase in n for one degree C. 
is in the mean about ‘00045. Asarule this difference is not 
constant, but increases with the temperature; in a few cases, 
however, as for instance ethyl isobutyrate and isobutyl buty- 
rate there are no second differences. Although the differences 
vary from ‘00041 to 00059, they seem quite irregular, and it 
is not easy to connect them with any chemical property of the 
bodies in question, and still more difficult is it to account for 
the appearance of the second differences in some cases and in 
others not. But I shall not attempt a further explanation of 
these points here. 
Of the above-named ethers the refractive indices of propyl 
acetate alone have been determined, as far as I am aware. 
Briihl gives for this for a sample boiling between 99°-101° the 
index np=1'38488 at 20°, and for another portion boiling from 
97°-99°ny=1'33360. It will be noticed that the first of these 
values corresponds very closely with that obtained by me. 
Indeed the agreement is better than one might expect, when 
the great liability of the ether to dissociation is taken into con- 
sideration. Briihl mentions this fact, and I have reason to 
believe that the sample used by me had likewise been slightly 
decomposed by the fractional distillation. 
In order to show the results obtained above, in their most 
general aspect, as well as for the purpose of better comparison 
with the work of others, I have arranged them in a more com- 
plete form below. In the columns headed d are contained the 
densities of the substances for each degree of temperature, cal- 
culated as explained at the outset. Under n are given the 
refractive indices to four places, the somewhat uncertain fifth 
% 
is given the “ specific refrac- 
tive energy,” and under M “: the “molecular refractive 
energy” for each compound at the various temperatures. M 
represents the molecular weight of the ether in question. 4 Use 
these terms in the sense in which they have been employ ed by 
Dale & Gladstone and Landolt, and ‘they need no further eX- 
planation. Propyl acetate I here omit, as I do not know the 
variations in its density with the temperature. 
The quotients — q 2re on the whole quite satisfactory, 
place being omitted. Under 
8 
been already noticed by Landolt and Wiillner,* yet it still 
remains to be explained. In the above ethers it will be noticed 
| 
: 
4 
q 
