34 Lieut. Hunt on Cohesion of Fluids, 



for its full development and illustration. I could not here 

 give all which belongs to it without exceeding reasonable 

 limits. Nearly all the views which I have presented were 

 the result of my own studies, so far as concerned my original 

 acquaintance with them, but I was happy to find that Donny 

 and Henry had, in some points, reached the same conclusions 

 by independent routes. But I am not aware that any one 

 has presented the same analysis of cohesio nor of the mole- 

 cular constitution of material surfaces. Especially does the 

 derivation of evaporation from molecular mechanics seem to 

 me novel and worthy of careful consideration. Donny indi- 

 cates essentially deaeration as a cause of steam-boiler explo- 

 sion ; but it is as an experimental deduction, and not con- 

 nected with its mechanical derivation. 



In conclusion, I will present an outline of a most interest- 

 ing illustration of creative design in the earth's co-ordination. 

 The explanation of evaporation which has been given shews 

 that for each fluid the formation of vapour lies within certain 

 definite limits of temperature, as a result of primary struc- 

 ture. These limits differ greatly in different fluids. Now, 

 in framing the earth for habitation, or for the proper life of 

 animal and vegetable forms, something equivalent to rain 

 was necessary, from the constant descent of fluids to the 

 lowest level. "Without some agency to lift the great organic 

 fluid above its lowest ocean bed, sterility would have been 

 the lot of all which rose above its surface, and terrestrial 

 organisms would have been quite impossible. But fluidity 

 does not involve evaporation except within certain definite 

 limits, special for each liquid. Again, evaporation might 

 freely go on, and yet no capacity for condensation exist, ex- 

 cept within other limits of temperature, quite unattainable, 

 save through special arrangement. Rain, then, with our 

 earth and atmosphere, involved a special constitution of the 

 raining fluid, not only so that evaporation at ordinary tem- 

 peratures should go on, but so that condensation may again 

 take place in the ordinary air. Not only must this qualitative 

 arrangement exist, but also a quantitative one ; since the 

 quantity of rain best sufficing to the aggregate organic need 

 is exactly a certain definite number of inches per annum. 



