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XX.—Photographs of Electric Sparks in Hot and Cold Air. By 
Professor Tair. (Plate XX XIV.) 
At the meetings of the Society on February 1 and 15, 1875, I exhibited a . 
number of photographs which had been executed for me in my laboratory by 
Mr A. Maruesoy, one of my students who has acquired great skill in working 
with very sensitive films of collodion, My object in advising him to undertake 
this work was to discover, if possible, the cause of the peculiar zig-zag form 
which electric sparks, and specially those of a Hotrz machine, always show 
in ordinary air. At the desire of the Council, a selection of a few of the more 
interesting of these photographs has been printed (by Mr Dattas, F.R.S.E.) 
for the Transactions from the original glass negatives, by what is commonly 
called the carbon process. 
Mr Marueson used a quartz lens which I had procured for experiments on 
Fluorescence. It was usually placed so as to give an image about half the 
dimensions of the spark itself; sometimes a little larger. The sparks were 
furnished by a double Hotrz machine made by Ruumxorrr. It was kept in 
perfect order by placing below it an inverted box of sheet iron, inside of which 
three small Bunsen lamps were kept burning. 
The first eight sets in the plate were taken in ordinary air. 
Figures 9-12 inclusive were taken in free air, a foot or two over the flame of 
a powerful 6-barrelled Bunsen burner. 
13-16 inclusive were taken in a wide glass tube, through which air had 
been passed (under slight increase of pressure) from a long narrow spiral tube 
of iron, kept at a dull red heat. 13 represents the result just after the current 
of hot air began to pass; 14 after a few minutes; 15 and 16 after nearly half 
» an hour. 

The first thing to be observed is, that in the hot air the sparks form in 
general much smoother curves than in the cold. A specially excellent instance 
_ of this is furnished by the two sparks in 9, taken at a fraction of a second 
interval, of which the upper passed through the ascending column of very hot 
_ air, while the lower (as is easily seen by its being partially out of focus, as well 
as being much more luminous) passed beside, but not through, the hot column. 
In various sparks of the first eight groups there are cases of sudden change 
of direction apparently through more than a right angle. To make absolutely 
VOL. XXVII. PART III. dU 
