290 FOURTH REPORT — 1834. 



a sufficient length of time, might he changed to concavity. 

 M.Poisson ad(hices a communication from M. Dulong containing 

 the following satisfactory explanation of this phsenomenon. In 

 the operation of boiling, a thin layer of the mercury in contact 

 Avith the air is oxidized, and then mingling with the whole mass, 

 changes its properties in such a manner, that the action of the 

 mercury on its own particles and on those of the tube, or rather 

 on the particles of a thin coating of water which is always in- 

 terposed between the mercury and the tube, is not the same as 

 before, the change being greater in proportion to the greater 

 quantity of metal oxidized, that is, in proportion to the duration 

 of the boiling. 



A formula obtained in a previous part of the work (Art. 53,) 

 applicable to the rise in a capillary tube of a fluid consisting of 

 two fluids mixed in given proportions, is here compared with 

 expei-iments made a long time since by Gay-Lussac,butnotbefore 

 published. This formula is founded on the supposition that the 

 loss of heat Mhich takes place in mixing, has no influence, 

 when the temperature has become the same as before, on the 

 integral which determines the value of H, and on which the 

 pliJEnomena of capillarity depend, an hj^othesis favoured by the 

 fact, that in the case of a single fluid, the decrement of elevation 

 at difi"erent temperatures is proportional to the augmentation of 

 density. The theoretical heights agree much less exactly with 

 the experimental for a mixture of water and alcohol, than for a 

 mixture of water and nitric acid; which shows that the above 

 hypothesis is more true for one mixture than the other. 



M. Poisson lastly applies his theory to the explanation of the 

 remarkable phaenomenon of endosmose. He conceives that the 

 two fluids meet without mi.ri)ig in the capillary tubes which 

 permeate the membrane, and by the relation of the molecular 

 forces at their common surface of separation, one prevails over 

 the other, and so passes through to the opposite side of the 

 membrane. It has been objected to this theory that it does 

 not account for the phaenomenon of exosmose*. An abstract 

 of Mr. Power's views on this subject having been inserted in 

 the Tldrd Report of the British Associatioit, it will be only 

 necessary to state that in the paper on Residuo-Capillary 

 Attraction by the same author, subsequently published in the 



• Admitting, as suggested by Professor Henslow, that each fluid tends to 

 spread into the other through the capillary communications, may not effects such 

 as are observed, be expected to result merely from the state of compression of 

 the fluid stratum in contact with the substance of the membrane? This will 

 vary with the varying density of the fluid from one point to another of the 

 surface of contact, being greatest where the density is least. 



