IS MA r'ri-:K ixDESTRcr'rir.LE 



i;\ II. 



TANLhY l\l.|)(.Ki)\i:. P..S( . (LoNi..!, !•.( .S. 



Whi-:n a candle burns it ceases to exist as such. 

 Closer examination of the phenomenon, however, 

 shows that this is not all that occurs. Not onlv 

 does the candle disai)pear. but some of one of the 

 constituents (oxygen) of the atmosphere is used up : 

 and in the place of the candle and oxygen, new gases 

 (carbon-dioxide and water-vajiour) make their 

 appearance. If all these bodies are carefully weighed 

 at the same spot on or above the earth's surface, it 

 will be found that the combined weights of the 

 carbon-dioxide and the water produced are exactly 

 equal to the combined weights of the candle and 

 oxygen consumed. A similar statement holds good 

 of every other chemical change ; the combined 

 weights of all the bodies produced during such a 

 change is alwa\s found to be exactly equal to the 

 combined weights of all the bodies consumed. 



Now, the weight of a body is the force by which 

 it is attracted to the earth's centre ; thus, the force 

 pulling a two-pound weight to the earth's centre is 

 twice that acting on a one-pound weight at the same 

 place. Force is sometimes defined as that which 

 produces or tends to produce motion. It appears, 

 however, that force is one of man's primary concep- 

 tions, and as such, is undefinable. since the idea of 

 force cannot be resolved into simj)Ier ideas; but even 

 as a description of force or of its effects the above 

 statement is not altogether satisfactory. In order to 

 keep a body moving with uniform velocity over a 

 rough surface, force must be continually applied to it. 

 But this force is needed to overcome friction, itself a 

 force which constantly tends to decrease the rate of 

 the body's motion — that is, to impart to it a retarda- 

 tion or negative acceleration. If more force than 

 that required to overcome friction is constantlv 

 applied to the body, it will move with ever-increasing 

 velocit\- — that is to say, the force will impart 

 acceleration to the body. Moreover, with the reduc- 

 tion of friction by any means, such as by the use of 

 a lubricant or by replacing the rough surface by a 

 smooth one, less force is needed to keep the body 

 moving with a given uniform velocity. The con- 

 clusion is justified, therefore, that could a perfecth- 

 smooth body be procured, no force would be needed 

 to keep it moving with uniform velocit\- over a 

 perfectly smooth surface ; though force would be 

 needed to start the bod\- so moving — that is, to 

 impart the acceleration to the body necessarv to 

 increase its velocity from i^ero to that velocity w ith 

 which it is required to move. It follows, therefore, 

 that force may be more accurately described than 

 bv the (letinition already given as that which 



produces or tends to produce acceleration (either 

 positive or negative), or what is the same thing, 

 chaiif^e of motion. 



If no forces whatever are operative on a body, it 

 will remain in a state either of rest or of uniform 

 motion : to change this state force is necessar)-. 

 This fact is expressed by saying that the body 

 possesses inertia. Inertia may, therefore, be defined 

 as that property of a body in virtue of which it tends 

 to keep in a state either of rest or of uniform motion, 

 and will remain in this state unless and in so far as 

 force is applied to it. Now, in order to produce a 

 given acceleration (or change of state of rest or 

 uniform motion) in different bodies, experiment 

 shows that different forces are necessary. Thus, 

 in order to accelerate two pounds of iron one foot 

 per second per second (that is, to increase its 

 velocity one foot per second every second), twice 

 as much force is necessary- as is needed to accelerate 

 one pound of iron one foot per second per second. 

 This fact is expressed bj* saj'ing that the inertia of 

 two pounds of iron is twice that of one pound of 

 iron. That is to say, the inertias of bodies may 

 be measured by applying to them (for the same 

 period of time in each case) such forces as are 

 necessar}- to impart to them a given acceleration : 

 the forces applied will then be jiroportional to the 

 inertias of the bodies. 



Now, if various bodies are dropped from the same 

 heights above the earth's centre, and if the friction 

 due to the air is obviated, by using a vacuum or 

 otherwise — that is to say, if various bodies are allowed 

 to move under the influence only of their respective 

 weights — it will be found that they will all fall with 

 the same constant acceleration, no matter what their 

 size, shape or weight mav be.* This acceleration 

 is called ^, and is, approximately, thirty-two feet per 

 second per second. Since the only force operative 

 on each body is its weight, and since the acceleration 

 is in all cases the same, it follows that the weights 

 of bodies determined at the same point relative to 

 the centre of the earth are proportional to their 

 inertias. 



It has alread\' been pointed out that the sum of 

 the weights of all the bodies produced b\' a chemical 

 change is exactly equal to the sum of the weights of 

 all the bodies consumed therein, so long as all the 

 \\eights are determined at the same place on or 

 above the earth's surface. If in place of " weights " 

 in this statement the word " inertias" is substituted, 

 the reservation may be deleted, and the inductive 

 law ma\" be formulated that chemical action has no 



''■'■ This may be demonstrated by means of the following very simple experiment. Place a small piece of paper on top of a penny 



(the paper must be smaller than the coin! ; allow them to fall together; the result will be that they will both reach the ground 



at the same time. The reason for placing the paper on top of the coin is that by this means the paper is protected from 



the retarding influence of the air, which is nuich greater in the case of the paper than it is in that of the coin. 



292 



