158 THE POPULAR SCIENCE MONTHLY. 



shall take the form of heat or give forth an electric current. In the 

 former case he shall connect the receivers together by suitable tubes, 

 apply a spark, and obtain a flame hot enough to fuse and vaporize 

 iron or platinum. In the other case he can use Grove's gas-battery, 

 and permit the elements to unite into water, producing an intense 

 electric curi'ent capable of working scores of miles of telegraph. The 

 realm of light yields us examples analogous to those given in the do- 

 mains of heat and electricity. In photography it has been discovered 

 that blue rays may begin an impression which red or yellow ones can 

 finish, and finish only. The power of continuance is diiferent from 

 the power of initiation, and depends upon it for its opportunity of use- 

 fulness. 



The instability of equilibrium among forces brings in an element 

 of uncertainty, or rather incalculability, which renders prediction ex- 

 tremely difiicult in many fields of scientific investigation. Prof. Bal- 

 four Stewart, in a most instructive essay on " Solar Physics," gives us 

 some illustrations of this. He supposes a stratum of air in the earth's 

 atmosphere to be very nearly saturated with aqueous vapor ; that is 

 to say, just a little above the dew-point ; while at the same time it is 

 losing heat with extreme slowness, so that if left to itself it would be 

 a long time before moisture were deposited. Now, such a stratum is 

 in an extremely delicate state of molecular equilibrium, and the drop- 

 ping into it of a small crystal of snow would at once cause a remark- 

 able change of state. For what would happen ? The snow would cool 

 the air around it, and thus moisture would be deposited in the form 

 of fine mist or dew. Now, this deposited mist or dew, being a liquid, 

 and as such much more radiant than vapor, would send its heat into 

 empty space much more rapidly than the saturated air ; and therefore 

 it would become colder than the air around it. Thus, more air would 

 be cooled, and more mist or dew deposited ; and so on until a com- 

 plete change of condition should be brought about, resulting perhaps 

 in a shower of rain. Now, in this imaginary case, the tiniest possible 

 flake of snow has pulled the trigger, as it were, and made the gun go 

 oif has changed completely the whole arrangement that might have 

 gone on for some time longer as it was, had it not been for the advent 

 of the snow-flake. Prof. Stewart thvis points out that the presence of 

 a condensable liquid in our atmosphere adds an element of violence, 

 and also of abruptness, amounting to incalculability, to the motions 

 which take place. Hence meteorology must long, if not ever, remain 

 an incomplete science, since in its problems so many variable and un- 

 stable factors occur. In the course of the same essay Prof. Stewart 

 tells us how parallelism has been observed between three very inter- 

 esting classes of phenomena, namely, the periods of maximum sun- 

 spots, of brilliant auroral displays, and of great disturbances in the 

 earth's magnetism. Extended observatory records show that all three 

 coincide in their fluctuations ; hence endeavors have been made to 



