282 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Dec, 1904. 



that the amount of material in a body can be measured 

 by its iccighi. If any other kind of sensitive balance 

 had been employed it would soon have been realised 

 that this is not legitimate unless it be assumed that 

 the quantity of material in a body varies according to 

 the part of the earth on which it happens to be placed. 



If weight, then, is not a satisfactory measure of the 

 quantity of matter in a body, can we find any property 

 that is? 



If we were concerned only witli matter of the same 

 chemical kind — for example, iron — and exactly the 

 same in all respects except that there was a bigger 

 volume of one than of the other, nobody would hesitate 

 to measure the quantity by volume. Matter would be 

 bought by the cubic centimetre, or cubic foot, or the 

 quart. Two quarts would always represent twice as 

 much matter as one. But the world is " full of a 

 number of things," and it is not easy to explain exactly 

 what you mean if you say that there is as much material 

 in a certain volume of water as in a certain block of 

 lead. What is the criterion of equality in such a case as 

 this when the material is of different kinds? In com- 

 merce, a certain volume of iron is given in exchange for 

 a much smaller volume of gold. In some sense, then, 

 these different volumes are taken as being a measure 

 of equivalent quantities of the two materials; and if a 

 fixed relation were preserved between these quantities, 

 a perfectly sound scientific system could be founded on 

 such a basis of equivalence. But familiarity with 

 market fluctuations would soon breed contempt for 

 such a system; it would be absolutely of no use for 

 scientific purposes. It has been agreed to measure 

 quantity of material not in the commercial way ; not 

 even by its weight, which is nearly satisfactory ; but by 

 another dynamical way, which, at any rate, till re- 

 cently, was thought to make the principle of the con- 

 servation of material precisely true under whatever 

 circumstances the quantity of matter is measured. 



We will explain this method. 



When Sir Isaac Newton thought out his Laws of 

 Motion, he percei\ed that every change of motion is 

 brought about by the influence of one body upon 

 another, and, moreover, that this influence is a mutual 

 one. When two billiard balls strike, the velocity of 

 both is changed; each influences the motion of the other. 

 A horse gives motion to a cart, but the cart simul- 

 taneously retards the motion of the horse. The main 

 part of the motion of the moon is controlled by the 

 influence of the earth, and reciprocally the moon modi- 

 fies the motion of the earth. 



Think now only of the simplest possible case, viz., 

 that in which the mutually influencing bodies move 

 along the same straight line; two billiard balls, for 

 example, moving without spin. When they strike, the 

 speed of one is increased and that of the other is then 

 always retarded. Measure, or (since this is not always 

 easy to dp, and we do not wish to introduce here the 

 complications of actual measurements) imagine 

 measured, the change of velocity of each. The ratio of 

 the changes, _,so far as all experiment has succeeded 

 in obtaining it, is_ the same for the same two bodies 

 whatever the previous velocities may have been. For 

 two billiard balls it would usually come out as 

 numerically equal to unity; whenever it does so the 

 masses of the miJucimng bodies are defined as being 

 equal. If the ratio of the changes of velocity is not 

 unity, the ratio of the masses of the tivo bodies is defined 

 as being inversely as the ratio of the changes of velocities, 

 whatever it may be. For example, if the balls be called A 

 and B, and A increase its velocity (due to the influence 



of B) from 4 to 10 units, while B diminish its velocity 

 (due to the influence of A) from 6 to 2 units, we have 

 Increase of velocity of A 6 _ Mass of B . 



Decrease ot velocity of 13 4 Mass of A. 



Hence in this hypothetical case B has ih times the mass 

 of A, and no matter what the circumstances of the 

 influence may be, this relation is found to be constant 

 for the same two bodies A and B; for example, if A's 

 velocity increase by 12 units due to B, then B's will 

 decrease 8 units due to A. The more massive body 

 has its velocity changed to the less degree; hence, mass, 

 as we have defined it, is a measure of the reluctance of 

 the body to be disturbed. A fly alighting on a cannon 

 ball scarcely affects the motion of the latter; a cannon 

 ball striking a fly sweeps it apparently irresistibly 

 before it. In each case the motion of the ball is 

 changed but little because its mass is so enormous in 

 comparison with that of the fly. 



The supposed constancy of the mass of a body under 

 every condition makes the mass an eminently suitable 

 means of measuring the quantity of material in it, and 

 is universally adopted as such. Thus, in the example 

 given above, B is said to contain i\ times as much 

 matter as A. This mode of measuring matter corre- 

 sponds to considering the quantities equal when the 

 same kind of substance is present m equal volumes; 

 but for different kinds of substances the quantities may 

 be equal when the volumes are very different. A cubic 

 foot of lead has about 11.35 times as much matter or 

 mass in it as a cubic foot of water. 



In a later number we will show the relation between 

 mass and weight, and we will then be in a position to 

 explain the nature of the evidence that has in recent 

 years been brought forward in the endeavour to prove 

 that the principle of conservation is not precisely true. 



The Radio-activity of 

 CKemical Reactions. 



By A. F. Burgess .AM) B. Ingram, B.A., F.C.S. 



It is, ol ctnu'se, a well-known fact that e\ery chemical 

 reaction is attended by the evolution or absorption of 

 energy in some form or another. That a chemical 

 equation does not adequately represent a chemical re- 

 action has recently been further attested by the work 

 of Colson, who has found that when a supersaturated 

 solution of sodium sulphate is made to crystallize it 

 gives out n,-rays. A mixture of aluminium, sulphate, 

 and potassium sulphate does not give out Uj-rays until 

 crystallization [i.e., with formation of alum) is started. 



Much interesting research has been done on this sub- 

 ject, and the more substantial proof has been supplied 

 by Landolt and Heydweiler, who have succeeded by 

 means of a balance of high precision in detecting 

 losses in chemical reactions by radio-activity. In fact, 

 the discrepancies, which Stas constantly encountered 

 in his weighings when engaged in his classical research 

 on the "Indestructibility of Matter," may now be 

 satisfactorily accounted for on the proof that, in chemi- 

 cal processes, the loss is due to certain emanations. 



Our first experiment (which was entirely specula- 

 tive) consisted of placing in a light-tight box the 



