68 PETER GUTHRIE TAIT 



in a circle of about eight inches diameter. Turning that through 90 from a position 

 perpendicular to the dipping needle, I got sufficient deflections of the galvanometer to 

 throw the light off the scale. My own peculiar experiments on light, which you 

 assisted at two years ago, I have arranged to try the very first fine day, and now with 

 some hope of success, although Thomson is not at all sanguine about the idea. 



I intend to repeat (if true) Tyndall's observation on the Adiathermancy of Ozone 

 with an instrument far superior to his. Perhaps something may come of it. 



The invention of the Divided Ring Electrometer indeed opened up many 

 new lines of research ; and in 1862 Tait and Wanklyn 1 published a joint paper 

 on the electricity developed during evaporation and during effervescence from 

 chemical action (Proc. R. S. .), in which attention was called to the large 

 charges produced by the evaporation of a drop of bromine and especially a drop 

 of aqueous solution of sulphate of copper, from a hot platinum dish. 



On January 23, 1862, in a letter mainly taken up with the projected 

 treatise on Natural Philosophy, Tait again got into ecstasy over Thomson's 

 galvanometers and electrometer. 



" They are splendid instruments. If you are in no hurry I will be over in Belfast 

 in April or May and will set them up for you. It requires some practice, but the gain 

 in visibility to the class is ENORMOUS. I showed by his electrometer today to my 

 whole class (150) in lecture the tension of a cell without condenser or anything of the 

 sort." 



On July 7 of the same year Tait mentioned the visit of Stas of Brussels 

 to Edinburgh and referred to experiments which he was doing along with 

 Wanklyn. With the preparation of the great treatise on hand, and the 

 consideration of the experiments on the rotation of a disk in vacua which 

 Balfour Stewart and he had begun upon, there was not much time for under- 

 taking any other experimental work on his own account. Tait was moreover 

 at this time working hard at quaternions. One very fruitful piece of experi- 

 mental illustration we owe, however, to this period. 



As will be more clearly brought out in the chapter on quaternions, Tait 

 was greatly impressed with Helmholtz's famous paper on vortex motion, so 

 much so that for his own private use he took the trouble of making a good 

 English translation of it. Early in 1867 he devised a simple but effective 

 method of producing vortex smoke rings ; and it was when viewing the 

 behaviour of these in Tail's Class Room that Thomson was led to the 

 conception of the vortex atom. In his first paper on vortex atoms presented 



1 Dr J. A. Wanklyn was assistant to Lyon Playfair the Professor of Chemistry. He was 

 a well-trained chemist, ingenious and resourceful. 



