5o8 POPULAR SCIENCE MONTHLY. 



from below. Vertical exchanges of temperature also take place 

 between higher and lower layers of the atmosphere, while the dif- 

 ference of temperature between the polar regions and the equa- 

 torial zone results in the assimilative movements of the general 

 atmospheric circulation. As ocean currents, like the Gulf Stream 

 of the Atlantic and the Kuro Siwo of the Pacific, carry heat from 

 the warmer to the colder regions of the earth, so air currents arise 

 out of those differences of heat and of barometrical pressure which 

 it is obviously their function, as far as possible, to remove. All 

 atmospheric movements, in fact, however local or general their 

 character may be, are either movements of direct assimilation by 

 which the atmosphere is seeking, so to speak, to bring all its areas 

 into like temperature and pressure with each other, or are dis- 

 turbances involved and arising indirectly out of such acts of as- 

 similation. It is only because the work done by these movements 

 is being constantly undone through the agency of influences, per- 

 manent and temporary, that differentiate areas of the atmosphere 

 in every part of the globe setting up, for example, unlikenesses 

 of temperature and pressure between the equatorial and polar 

 regions, between continents or islands and the surrounding oceans, 

 or between any area of the earth's surface abnormally heated or 

 cooled and the surrounding parts of that surface, as well as be- 

 tween seasonal variations in such inequalities that we have cy- 

 clones and anticyclones, tornadoes, blizzards, land and sea breezes, 

 mountain and valley winds, sand spouts and dust whirlwinds, as 

 well as various periodical and more or less local disturbances all 

 over the world. It should be added that meteorological phenom- 

 ena do but illustrate the wider interchanges that take place in 

 the ether system, since the constant distribution, as electro-mag- 

 netic disturbances, of movement differentially accumulated in 

 material aggregates whether such disturbances take place within 

 purely local limits, as in circuits artificially set up, or on a uni- 

 versal scale, as by diffusion from solar bodies are all cases of the 

 distribution of movement, and therefore cases of assimilation. 



The diffusion of molecules through each other is also a com- 

 mon form of assimilation. Gases, if brought together, permeate 

 each other until a tolerably like constitution for every larger or 

 smaller area of the total volume has been reached ; gas distrib- 

 utes itself equably through liquids, as in the case of effervescing 

 drinks; solutions of salts brought into contact gradually inter- 

 mingle. A soluble solid, when introduced into a liquid, usually 

 assumes the liquid state to the extent of the capacity of the fluid 

 for taking it up, as in the familiar case of sugar in tea or alum 

 in water, while the liquid itself undergoes modification by the 

 equable distribution of the particles absorbed. The uniform 

 hardness of " hard " water, due to the presence of bicarbonate 



