268 ATOM 



For instance, I have already said that two grains of water saturate a cubic 'foot of air 

 at 30 ; if, therefore, the temperature of the air bo 40, .and there bo two grains of 

 moisture in a cubic foot of air, then if the bulb of the hygrometer bo reduced to 30, 

 a ring of dew will appear on it, caused by the deposition of the .water in the air. 

 The determination of the dew-point at once tells us, therefore, the amount of water 

 present, and, combined with the temperature, enables us to determine the hygrome- 

 trical state of the atmosphere. 



' If the air be saturated with moisture, the temperature of the air and that of the 

 dew-point are alike ; if it bo not saturated, the temperature of the dew-point is lower 

 than that of the atmosphere ; if there bo a great difference between the two tempera- 

 tures, the air is dry, and if this happen when the temperature is low, there is very 

 little water present in the air. By the careful simultaneous readings of two thermo- 

 meters, one with a moistened bulb, and the other dry, or by the use of a Darnell's or 

 Eegnault's hygrometer, the amount of water present in the air in the invisible shape 

 of vapour can be determined,. as well as the temperature of the dew-point and the 

 degree of humidity. 



' The degree of humidity of the air expresses the ratio between the amount of water 

 then mixed with it, and the greatest amount it could hold in solution at its then 

 temperature, upon the supposition that the saturated air is represented by 100, and 

 the air deprived of all moisture by 0. Thus : Suppose the water present to bo 

 one-half of the quantity that could be present, the degree of humidity in this case 

 will be 50. If the air were at the temperature of 30, and there were two grains of 

 moisture in the air, it would bo saturated, and the degree of humidity would be 100. 

 If there were one grain, that is, one-half of the whole quantity that could bo 

 present, the air would be one-half saturated, and the degree of humidity would be 

 represented by 50. 



1 At 49 with 4 grains of moisture 1 ,, . . 



- O o o I The air is saturated and the 



degree of humidity is 100. 



But at 49 with 2 grains of moisture "1. The air is one-half saturated, 

 ' " ' " " and the degree, of humidity is 50.' 



I 



ATOM, (a, not; re'/ww, I cut.) An indivisible particle. 



With few exceptions, the views promulgated by Dr. Dalton are received by chemists. 

 They may bo thus expressed : All elementary bodies are formed of individual atoms, 

 the different species of which unite, generally by twos, in a small number of groups, 

 constituting compound atoms of the first order, always mechanically indivisible, but 

 chemically divisible, and, in their turn, constituting all the other orders of composition 

 by a series of analogous combinations. 



We are not enabled by direct experiment to determine the condition of any ultimate 

 atom of matter ; but the results furnished by chemical science clearly point to the 

 existence of elementary units, from which all the infinite varieties of matter aro 

 formed. Sir Isaac Newton thus expresses himself : ' All things considered, it seems 

 probable that God, in the beginning, formed matter in solid, massy, hard, impene- 

 trable, movable particles, of sucli sizes, figures, and with such other properties, and 

 in such proportions to space, as most conduced to the end for which He formed them ; 

 and that these primitive particles, being solids, aro incomparably harder than any 

 porous bodies compounded of them ; even so hard as never to wear or break to 

 pieces ; no ordinary power being able to divide what God Himself made one in the 

 first creation. While the particles continue entire, they may compose bodies of one 

 and the same nature and texture in all ages ; but should they wear away, or break in 

 pieces, the nature of things depending on them would be changed. Water and earth 

 composed of old worn particles would not be of the same nature and texture now 

 with water and earth composed of entire particles at the beginning. And therefore, 

 that nature may be lasting, the changes of corporeal things aro to bo placed only in 

 various separations, and new assocmtions, and motions of these permanent particles ; 

 compound bodies being apt to break, not in the midst of solid particles, but where 

 those particles are laid together and touch in a few points.' Horslei/s Newton. 



With the metaphysical theories, which would lead us to regard all matter as mere 

 accumulations of force, it would not be proper at present to deal. 



Experimental philosophy has proved to us that the conditions of matter are deter- 

 mined by certain polar-attractive forces ; and that these are opposed or balanced by 

 heat, electricity, and the force which regulates chemical combination. Consequently, 



