NATURAL PHILOSOPHY. 171 



but rather forces balanced against each other, acting indeed in 

 antagonism." 



THE PHYSICS OF ABSORPTION. 



The curious fact pointed out by Pouillet, in 1822, that when a 

 fluid is absorbed by a porous substance a rise in temperature oc- 

 curs, has given origin to some strange exi^hinations and discus- 

 sions. The subject has I'ecently been taken up by Jungk, who 

 attributes the alteration in temperature to the foniiation around 

 each particle of the poi'ous body of a thin layer of fluid, "in which 

 the individual molecules move with much less freedom ; thus point- 

 ing to a condensation of the fluid in those parts." In sup2:)ort of his 

 theory, he quotes a paper by Rose, on the errors which arise in the 

 detei'mination of the specific gravity, when the substance is weighed 

 in a state of fine subdivision. The finer the particles of the body 

 under examination, the greater will be the resulting sjiecific gi'av- 

 ity. He proceeds by assuming that the temperature of a body i-ises 

 or falls when, by any external means, it is caused to assume the 

 condition induced by the subtraction or addition of lieat respect- 

 ively. Ai32:)lying this in the case of water, it would follow that, 

 when absorl^ed by a porous substance, the temperature should 

 either rise or fall according as the water is below or above four 

 degrees Centigrade, — the point of maximum density. This, in 

 fact, was found to be the case ; and the results of his experiments 

 maybe shortly stated as follows: 1. Tlie temperature of water, 

 when absorbed by sand, is raised or lowered according as it was 

 previously either above or below four degrees C. 2. Water at 

 zero, when absorbed by snow, is lowered in temperature. 3. The 

 plienomenon may be regarded as a consequence of the condensa- 

 tion of the water on the surface of the absorbent body. — Foggen- 

 dorff^s Annalen, 1865. 



LIFE-TABLES. 



A paper entitled " New and Compendious Method for the Con- 

 struction of Life and Annuity Tables from Returns of Population 

 and Mortality," was read by Mr. E. B. Elliot at the 1866 meeting 

 of the American Association for the Advancement of Science. 



He remarked that life-tables assume various forms, the more 

 common form being that which gives the number surviving dif- 

 ferent ages out of a given number of persons living at some 

 earlier age specified. 



The population of any community is usually a fluctuating one ; 

 it varies with the excess of births over deaths, and of immigra- 

 tion over emigration. 



A stationary population is one unafi'ected by migration, and in 

 which the births are equal in numljer to the deaths, the number 

 of persons annually entering upon any age ])eing equal to the 

 number of deaths which annually occur at and over that age. 

 Hence, if we have either the number of annual deaths or the 

 number of the living at different ages in a stationary population, 

 we have in desirable form life-tables for that population. 



