248 Mr. C. Tomlinson on some Effects of 



to the evaporation and condensation of the liquid in the glass. 

 This effect may be well shown by putting a little pyrolig- 

 neous ether or wood-spirit into a deep glass, covering it with 

 a receiver and exhausting the air. Under these circumstances 

 it readily boils, and the vapour condenses at various points of 

 the surface so as to form tears. The same effect may be 

 shown by filling a long tube with spirits of wine, and then 

 nearly emptying it, so as to clean the surface. If the tube be 

 now fixed vertically and the flame of a spirit-lamp be applied 

 below for a short time, tears will form during the cooling at dif- 

 ferent heights above the liquid. If spirits of wine be poured 

 into a glass and a portion be then turned out, tears will form, 

 but not nearly so well as in the case of wine. A little ether 

 added to the spirit improves the effect. The explanation in this 

 case is the same as in that of wine. Ether alone, or bisul- 

 phide of carbon alone, will not form tears, as the plate of liquid 

 instantly evaporates. Benzole does not act well ; naphtha acts 

 tolerably well. 



I have noticed tears formed by a very strong solution of the 

 sodic acetate (5 parts salt to 2 water), which was boiled at 

 234° F., and filtered into a chemically clean flask, in which it was 

 boiled again. It was then plugged with cotton-wool and left to 

 cool. At about 90° the flask was put into water at 40°, and 

 then into water at 32° containing snow. The glass above the 

 solution now began to weep, the ascending and descending cur- 

 rents being well marked on the chemically clean surface. 



The care bestowed by me upon chemically clean surfaces in 

 studying the phenomena of supersaturated solutions seems to 

 have greatly simplified what was a very complicated subject. 

 Take the case of supersaturated gaseous solutions, such as soda- 

 water, Seltzer-water, champagne, &c* If any one of these be 

 poured into a chemically clean glass no bubbles of gas will form 

 on the sides, because the adhesion between the sides and the 

 solution is perfect, and the sides may be regarded as a continua- 

 tion of the liquid itself. A clean glass rod placed in the liquid does 

 nothing more than form new sides, as it were, to the vessel, and 

 its effect is merely that of the sides. In such case the rod 

 will form no bubbles around it, and hence it is "inactive," 

 because its adhesion is perfect. If dirty, the surface of the liquid 

 surrounding it will be free ; that is, there will be little or no adhe- 

 sion between the water of the solution and the dirty surface ; but 

 there will be an adhesion between the gas of the solution and the 

 unclean surface, and hence there will be a liberation of gas. All 

 bodies that have been exposed to the air or to the touch are che- 

 mically unclean : they are covered with an organic film, as already 

 * Phil. Mag. for August and September 186/. 



