Magnetised Iron, $c., cooled to Temperature of Liquid Air. 57 



of gas is a mixture of these two, its refractivity will determine the 

 proportions in which the components are present. 



The observations were made by an apparatus similar in character 

 to that already described, but designed to work with smaller quan- 

 tities of gas. The space to be filled is only about 12 c.c., and if the 

 gas be at atmospheric pressure its refractivity may be fixed to about 

 1/1000 part, By working at pressures below atmosphere very fair 

 results conld be arrived at with quantities of gas ordinarily reckoned 

 at only 3 or 4 c.c. 



The refractivity found for the Bath residue after desiccation was 

 0*896 referred to air, so that the proportional amount of helium is 

 8 per cent. "Referred to the original volume, the proportion of helium 

 is 1 P 2 parts per thousand. 



" On the Changes produced in Magnetised Iron and Steels 

 by cooling to the Temperature of Liquid Air." By JAMES 

 DEWAR, LL.D., F.R.S., Fullerian Professor of Chemistry in 

 the Royal Institution of Great Britain, and J. A. FLEMING, 

 M.A., D.Sc., F.R.S., Professor of Electrical Engineering in 

 University College, 'London. Received April 25, Read 

 May 21, 1896. 



The action of the low temperature produced by liquid air upon the 

 magnetic moment of steel magnets was studied by one of us in a few 

 cases in a preliminary research made some time ago.* We have re- 

 cently returned to the subject and made further investigations on 

 the influence of the low temperatures thus obtained on magnetised 

 iron and steels of very various compositions, with the object of de- 

 termining the nature of the changes which take place in the 

 magnetic moment of small magnets constructed of these metals, 

 when cooled gradually or suddenly down to the lowest temperature 

 obtainable by the use of boiling liquid air. The arrangements adopted 

 in this investigation were as follows : 



A reflecting magnetometer consisting of three small magnetised 

 needles of watch-spring steel, cemented to a concave glass mirror, 

 suspended by a single cocoon fibre, was placed in a tube so as to be 

 free from disturbance by draughts of air. The small magnets were 

 8 to 10 mm. in length. The image of a portion of the filament of 

 an incandescent lamp was reflected by the mirror on to a divided 

 scale placed at a distance of 70 cm. from the mirror. The edge of 

 the very sharp image of the filament, focussed upon the scale, 



* Friday evening discourse at the Koyal Institution, "On the Scientific Uses of 

 Liquid Air," by James Dewar, LL.D., F.E.S., January 19, 1894. 



VOL. LX. F 



