432 
geneous plane polarised light in the direction of the lines of' force 
of the field, then the rofation which the plane of polarisation under- 
goes at a known temperature is the absolute magnetic rotation of 
the substance. 
Generalij we do not require the absolute value and the relative 
value with reference to a Standard substance is sufficiënt. 
Perkin, the veteran worker in this line, chose water as the Stan- 
dard substance. 
Hence denoting the specific rotation bj r we have the expression, 
r = -, where a.is the rotation of the given substance . ■ . M (mole- 
a 
a óm 
cular magnetic rotation) = , where m and d are respectively 
a d p 
the molecular weight and density of the substance and p and tf the 
corresponding values for the Standard (since in Perkin’s work, tubes 
of equal length were always used). 
The magnetic rotation of the plane of polarised light is measured 
in the same- way as the permanent rotation of a substance, but the 
apparatus is more complex, since an arrangement for placing the 
substance in a magnetic field is provided. The tube containing the 
liquid is placed either bet ween the poles or as in Perkin’s latest 
form of apparatus in the hollow cone of a powerful electromagnet. 
The chief precaution to be observed in addition to those of an 
ordinary polarimetric determination, is in preserving a constant, strength 
of the magnetic field. 
The rotations of the Standard and of the substance are measured 
in the same tube under identical conditions of temperature and 
magnetic intensity [Perken, Trans, chern. Soc. 421 (1884); 69 1025 
(1896); 89 , 605 (1906)]. 
From an exhaustive study of organic compounds, Perkin has shown 
that the addition of CH 2 causes an approximately constant increase 
in molecular rotation and this increase is very nearly the same in 
different classes of compounds. Perkin has calculated the average 
value for CH 2 from a wider range of material and he found that 
CH 2 = 1.023. 
If there are n CH 2 groups in a compound whose molecular rota- 
tion is M, then the expression M — n (1.023) = £ represents the 
rotatory effect of the remainder of the molecule. In a large number 
of organic compounds it is seen that S is approximately constant 
for all the higher members of a given series. Thus S is called the 
series constant. The series constant 0.508 of the normal paraffins 
Cn # 2/1 + 2 is obtained by subtracting the value of n CH 2 from the 
