aeneous plane |iulaii.sc(l liiilil in iIip directiDii of llie lines of force 

 of (lie fielil, ilien the rotaiioii which the plane of polarisation under- 

 goes at a known leniperatiue is the absolute magnetic rotation of 

 the substance. 



Generally we do not require the absolute value and the relative 

 \alue with reference to a standard substance is sufficient. 



Pekkin, the veteran worker in this line, chose water as the stan- 

 dard substance. 



Hence tlenoling the s|iecillc rotation by r we have the expression, 



r= , where a is the I'otation of the given substance .-. M (mole- 

 st 



a (i III . 



cular magnetic rotation) = — , where /;/ and d are respectively 

 « d ft 



the molecular weiglit and density of the substance and [i and rf the 



corresponding values for the standard (since in Fkrkin'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 between 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 fiekl. 



The rotations of the standard and of the substance are measured 

 in the same tube under identical conditions of temperature and 

 magnetic intensity [Pkrkin, Trans, chein. Soc. 421 (1884); 69 1025 

 (1896); 89, 605 "(1906)]. 



From an exhaustive study of organic compounds, Pekkin has shown 

 that the addition of CH^ causes an approximately constant increase 

 in molecular rotation and this increase is ver^- nearly the same in 

 diflferent classes of compounds. Perkin has calculated the average 

 value for (JH, from a wider range of material and he found that 

 CH, = 1.023. ' 



If there are n ClI, groups in a compound whose molecular rota- 

 tion is }[, then the expression M — a (1.023) = »S' represents the 

 rotatory etfect 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 

 ('n //■_>„ -1-2 is obtained by subtracting the value of n CH, from the 



