TRANSACTIONS OF SECTION A. 787 



netic force in the air or ether around our apparatus. Let now the frequency he • 

 raised to several hundred million million periods per second ; we shall have visible 

 polarised light proceeding as if from an ideal point-source at the centre and 

 answering fully to the description of the light of I. (2). The same result would 

 be obtained by taking simply a fixed solid non-conducting globe aud laying on wire 

 on its surface approximately along the circumferences of equidistant circles of lati- 

 tude, and, by the use of a distant source (as in II.), sending an alternate current 

 through this wire. In this case, while there is no manifestation of electrostatic 

 force, there is strong alternating magnetic force, which in the space outside the 

 globe is as if from an ideal infinitesimal magnet with alternating magnetisation, 

 placed at the centre of the globe and with its magnetic axis in our axial line. 



6. Heat of Combination of Metals in the Formation of Alloys. 

 By Alexander Galt, D.Sc. 



Hitherto few experiments have been made to determine the heat of combination 

 of zinc with copper, or of other pairs of solid metals. Not only in connection with 

 th'» theory of contact electricity in particular, but generally in respect to chemical 

 aftinities it is important that we should have some knowledge in regard to this 

 question, and at the request of Lord Kelvin I have carried out the following 

 experimental investigation in the physical laboratory of the University of 

 Glasgow. 



The method of procedure was to dissolve a known weight of an alloy and also 

 under similar conditions the same weight of a mixture of the elements wliicli are 

 present in the alloy, the proportions taken being the same as those known to be in 

 the alloy, and noting the initial aud final temperature in each case. The heat of 

 combination of the metals in the alloy mav be estimated after noting the excess of 

 the heat of solution of the mixture over that of the alloy. 



A large number of preliminary experiments were made to determine the mo.st 

 suit-.ible conditions for the invpsHa-ation. The nature and strfinc-iii of the solvent 

 and its quantity for a given mass of the metals to be treated, keeping in view 

 the advisability of obtaining a moderate range of thermometric readings and the 

 necessity of minimising as far as possible the violence of the reaction between 

 metals and solvent, had to be settled. 



Messrs. Johnson, Matthey & Co., of London, kindly made for me and analysed 

 five different alloys of practically pure zinc and copper, and they also supplied 

 separate specimens of zinc and copper similar to those used in making the alloys. 

 To facilitate solution the metals used were first reduced to powder by filing with 

 a fine file. The method of experimenting finally adopted was carried out in 

 detail as follows : One end of a short length of closed tliin glass tubing was 

 sealed to a very small glass bulb. Near the point of attachment there were, on 

 opposite sides, two ova.l-shaped openings in the bulb. The glass tube was free 

 to move up and down a certain distance througii one of two holes bored through 

 a short common cork. Special care being taken to see that the bulb war? clean 

 and dry, it was drawn down from the cork about six centimetres, and the cork 

 fixed in a clamp. The filings (5 gramme was the quantity always used in each 

 of the experiments) were then most carefully inserted into the bottom of the 

 bulb by one of the openings, and the bulb was then drawn up close to the 

 cork. Through the other hole in the cork a very thin sensitive short-range ther- 

 mometer, whose marked divisions correspond to 005° C, was passed. The cork 

 carrying the bulb and attached tube and thermometer was then carefully fixed in 

 tlie neck of a small flask of thin glass containing nitric acid of density LofiO. 

 ^0 rubic centimetres of which were used in nearly every one of these experiments. 

 Holding the fla.sk by the lip, it was gently shaken so as to give the acid a rotating 

 motion; in this way the flask and contents soon attained a uniform temperature, 

 which was carefully noted. The bulb was now plunged to near the bottom of the 

 flask by quickly pushing down the glass tube to which it was attached ; this plan 

 effectually got rid of the difficulty of escaping fumes previously experienced. At 

 the same instant the flask was inserted into an empty can, jacketed round the 



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