EECEXT PROGRESS IN RELATION TO THE THEORY OF HEAT. 237 



to an equal ])art of tlic internal Vdhor rx2)cnded, and the otlicr part will be equiva- 

 lent to the lieat made apparent, consequently, to the elevation of temperature. 

 This elevation will be less, therefore, than that of the prec(Hlino,- operation, and 

 the ditl'erence will be proportional to the external labor produced. 



But this operation may be reversed, and when we proceed in such manner as 

 to dihite the body mechanically, there is an internal labor [)roduced which rcmahis 

 greater than the external labor expended; the difFerence of these two labors cor- 

 resjionds to the disappearance of a pn)portional quantity of heat, hence a spon- 

 taneous lo'wcring of the temjierature, 



Thus experiments of this kind furnish us a relation between the external laV)or, 

 the internal labor, and the heat created or destroyed. On the other hand the 

 mechanical theory establishes a mathematical relation between these quantities. 

 It is jjracticable, therefore, to submit a conse(]nence of this theory to the test of 

 experiment. Such is the olyect which M. Edlung proposed to himself, and it 

 may be said that the verification has been as complete as possible. But it does 

 not appear possible to draw from such experiments the exact value of the 

 mechanical equivalent of heat, on account of the impossibility of preventing' the 

 calorilic influence of neighboring bodies. As the traction is not instantaneous, 

 neither can the thermometric effect be so ; the effect which we observe is there- 

 fore too small, and the theoretic formula which serves to calculate the mechanical 

 equivalent yields a value too great. If we establish a system of corrections in 

 regard to the effect of the surrounding bodies, the uncertainty is not less great, 

 because of the minuteness of the thermometric effect that is measured. 



By the side of the speculative researches which have aggrandized our knowl- 

 edge respecting heat within a few years past, of which I have been able to sig- 

 nalize but a small number, maybe ranged certain interesting experiments which 

 have been devised for the ])opularization of science, and Avith which most of us 

 are already familiar. I have selected one of those which we owe to the celebrated 

 English professor, M. Tyndall, because it is the reproduction of a striking natural 

 phenomenon. I refer to the intermittent eruptions of water and vapor met with 

 in Iceland. M. Binisen has furnished a very simple explanation of volcanoes of 

 this kind, which arc called geysers, and M. Tyndall has very ingeniously imitated 

 them. 



Imagine a pit of a depth of twenty metres, and a breadth of three; at the 

 bottom there is water heated by the volcanic substances which i)roceed from the 

 deiitli of the earth. The different strata of water occur underpressures increasing 

 fi'(jm above downwards, since each stratum nnist sustain the pressure of the atmo- 

 sphere and that of tlio column of water which is above it. Tlie temperature of 

 ebullition of these strata will therehjre increase, in lilte manner, from al)ove 

 downwards. Let us consider a stratum having a temperature a little below that 

 of its ebullition, under the conditions in which it actually exists: if its pressure 

 be diminished, it is thrown into ebullition. This is precisely what takes place 

 in the geyser. Aqueous vapor being formed at the bottom of the pit, where the 

 heat is strongest, lifts up the strata of water above. If one of them be raised 

 sutliciently high, it passes into a state of ebullition ; the water which is below it 

 is less compressed ; it boils in its turn, and a mass of vapor is instantly formed 

 at the bottom of the pit. Tliis vapor expels the upper strata of water, and itself 

 issues with them, forming an innneuse sheaf-like jet. The expelled vapor io 

 cooled, becomes liijuid, and falls back with the projected mass of water; by its 

 re-entry the temperature of the pit is reduced, and ebullition is suddenly arrested. 

 Vv'e now hear a Cfjncussion proceeding from the formation of new bubldes fs 

 water, because all the parts of the [lit are not instantaneously chilled; until 

 finally, repose is re-established. But the central hei»t gradually restores the c<tl- 

 umn to its former state, and a new eruption takes place. In the experimental 

 demonstration, the geyser is represented \)y a tube of metal, two metres in length, 

 Burmounted by a basin. It is filled with water, and tw > sources of heat are eslab- 



