ON THE STUDY OF BIOLOGY. 527 



in the volume of air through which it swept, the diameter of the drop 

 after passing through 2,000 feet would be more than an eighth of an 

 inch, and after passing through 4,000 feet a quarter of an inch, and so 

 on. So that, in passing through 8,000 feet of such cloud, it would ac- 

 quire a diameter of half an inch. 



The fact that raindrops never attain the size of large hailstones is 

 explained as being due to the mobility in the case of large drops of 

 the surface tension of the water, by which alone the drop retains its 

 form, to withstand the disturbing force of the air rushing past ; when 

 the drop reaches a certain size, therefore, it is blown in pieces like the 

 water from a fountain. 



The origin of stones and drops is then discussed why some of the 

 particles in a cloud should be larger than the others, as it is necessary 

 for them to be in order that they may commence a more rapid descent. 

 A cloud does not always rain ; and hence it would seem that in their 

 normal condition the particles of a cloud are all of the same size and 

 have no internal motion, and that the variation of size is due to some 

 irregularity or disturbance in the cloud. 



Such irregularity would result when a cloud is cooling by radiation 

 from its upper surface. The particles on the top of the cloud being 

 more exposed would radiate faster than those below them, and hence 

 they would condense more vapor and grow more rapidly in size. 

 They would therefore descend and leave other particles to form the 

 top of the cloud. In this way we should have in embryo a continuous 

 succession of drops. 



Eddies in the cloud also form another possible cause of the origin 

 of drops and stones. Nature. 



OX THE STUDY OF BIOLOGY. 1 



Br Pbofessoe T. H. HUXLEY. 



IT is my duty to-night to speak about the study of biology, and 

 while it may be that there are many among you who are quite 

 familiar with that study, yet, as a lecturer of some standing, it would, 

 I know by experience, be very bad policy on my part to suppose such 

 to be extensively the case. On the contrary, I must imagine that 

 there are many of you who would like to know what biology is ; that 

 there will be others who have that amount of information, but would 

 nevertheless gladly learn why it should be worth their while to study 

 biology; and jet others, again, to whom these two points are clear, 

 but who desire to learn how they had best study it, and, finally, when 



1 A lecture by Prof. Huxley, delivered at the South Kensington Museum, on Saturday, 

 December 16, 1876. 



