a Chemically Clean Surface. 251 



of clean platinum-foil is one of a large class of facts which also 

 require special treatment. 



There is a curious example of the production of chemically 

 clean lines on the surface of glass by the discharge of a Leyden 

 jar ; a square of window-glass, four inches to the side, that has 

 been exposed to the air so as to contract the organic film so often 

 referred to, or dipped into a strong aqueous solution of soap and 

 rubbed dry with a cloth so as to leave an invisible film of soap 

 on the surface, will answer the purpose. If such a pane be pre- 

 sented to the knob of a charged Leyden jar (not a large one) and 

 the discharging rod be placed with one knob on the outer coat- 

 ing and the other on the glass pane, the discharge will be made 

 first on the glass next the knob of the jar, and it will pass round 

 the edge and so get to the discharging rod. On holding the 

 glass up to the light no electrical effect will be seen ; but on 

 breathing upon the glass, a remarkable tree-like figure will be 

 traced out by the breath condensing in liquid lines wherever the 

 electricity has burnt away the film and left the glass chemically 

 clean, whereas on other parts of the glass, not touched by the 

 discharge and where the glass is not clean, the breath will be 

 deposited in minute beads of dew*. 



I have already pointed out in this Journal f that the texture 

 of dew depends greatly on the condition of the surface which 

 receives it as to chemical purity. If chemically clean bottles, 

 containing water and air or some gas, be placed near the window, 

 we do not get deposits of dew, but weeping tears trickling down 

 over sheets of water and leaving no furrows. In the article in 

 the English Cyclopaedia just referred to, I wind up with the fol- 

 lowing remarks : — " It is curious to notice the various modes in 

 which the breath condenses on the glass plates, according to the 

 temperature of the air and the amount of humidity or dryness, 

 the tension of the electricity, and the state of health of the 

 experimentalist. During the warm humid weather which oc- 

 curred on certain days at the end of May and the beginning of 

 June 1859, it was almost impossible to produce these lightning- 

 figures. Then, again, in breathing on the plates a quantity of 

 organic matter is deposited, which varies with the kind of food, 

 the time that it has been taken, and the state of health of the 

 individual. The breath, moreover, is differently deposited when 

 projected by different individuals. The state of the breath is 

 known to have a considerable influence in photography, and it 



* I have named these figures " Lightning-figures:" see Edinb. New Phil. 

 Journal for October 1861 and January 1862. See also the article " Breath- 

 figures " in the English Cyclopaedia, Arts and Sciences division, vol. i. 

 col. 313. See also letter to the 'Times ' newspaper, September 10, 1866. 



t Phil. Mag. for May 1863. 



