KDouiledge & Seieotilie Ileuis 



A MONTHLY JOURNAL OF SCIENCE. 



Conducted by MAJOR B. BADEN-POWELL and E. S. GREW, M.A. 



Vol. I. No. ii. [new series.] DECEMBER, 1904. 



[Entered at n 

 Stationers' Hall.J 



SIXPENCE. 



CONTENTS.-See Page IX. 



The ConservQLtion of 

 Mass. 



By Alikeu W". Porter, 



Fellow of, and Assistant Professor of Physics in, University 

 College, London. 



The principle which is the basis of all analytical 

 chemistry is expressed by saying that whatever com- 

 binations or separations are effected in different 

 materials the total amount of material present remains 

 constant. The amount of material referred to in this 

 statement is not measured by its volume ; indeed, in 

 many cases, this imdergoes considerable change. The 

 measurement is made by a balance, and the actual 

 process of measurement consists in counterpoising the 

 substances under examination — both in the free and 

 combined states — against given " weights "; the 

 " weight " so obtained is then taken as being a measure 

 of the quantity of material present. 



We are not now concerned with the illustration of 

 this very familiar principle, nor with the question of 

 its practical truth, which is undoubted. Every chemist 

 trusts in its truth when he expects the results of his 

 analysis to add up to 100 per cent. But as considerable 

 interest is to-day felt in the possibility that the law is 

 not completely satisfied, we intend to examine as simply 

 as possible the experimental means by which its failure 

 might be ascertained. 



However, in the first place, it is important to have 

 clear ideas as to what the problem really is. Some 

 confusion of thought about it is prevalent. This con- 

 fusion arises from the current confounding of mass 

 and weight. Can we take the weight of a body as 

 being proportional to the quantity of material in it? 

 My housekeeper tells me it is so. Two pounds of 

 sugar weigh twice as much as one, and there is twice 

 as much sugar in them. Unquestionably .so — from )ier 

 point of view; but we must look at things a little more 

 accurately than my housekeeper docs. 



When any material is placed in one pan of a balance 

 it presses on it with a certain force. This force is said 

 to be " due to gravity " — a statement, however, which 

 does not add much to our knowledge. It would be 

 much more explicit to say it is due to the earth; for 

 there is every reason to believe that if the earth were to 

 disappear the force would vanish too. It is this force 

 w-hich is scientifically defined as being the weight of the 



body. \ow if anything has been proved with certainty, 

 it is that the weight of a body is not always the same. 

 Hang it on a spiral balance; the extension of the spiral 

 spring is less if the experiment is made near the 

 i-quator than near the poles. Or, better still (for a 

 spiral spring is not very sensitve), place the body on 

 one of the extremely ingenious spring balances which 

 have been recently devised, and which consist simply 

 of a horizontal fibre of quartz supported at one end — a 

 cantilever, in fact. These are of wonderful sensitive- 

 ness and constancy also; the same force at the free end 

 produces the same amount of bending every time. 

 But a given fragment of material placed on it will pro- 

 duce a different deflection near the earth's equator than 

 if the experiment is made near the earth's poles. It 

 will be different if the apparatus is high up a mountain 

 than at the sea-level. There is no constancy of weight 

 c\en for the same body in the same state; this is 

 acknowledged by all. .So that, after all, it cannot be 

 the weight that is being taken as a measure of the 

 amount of material when the principle of the conserva- 

 tion of material is asserted to be true. The confusion 

 arises from a peculiarity of the ordinary hal;mce. The 

 material is placed in one scale pan iqjon which it 

 presses down. Weights (i.e., st.andard blocks of 

 material) are placed in the second pan, upon which 

 they press down. If the two just counterbalance one 

 another, and if the balance is accurate, they are said to 

 be equal to one another. The comparison made here is 

 between the turning power of two weights, and if the 

 arms of the balance are of the same length, equality 

 of turning power involves equality of the weights them- 

 selves. l-!\en in the most accurate analyses, then, 

 although it is the iveighis of the constituents of any 

 body which are currently taken as being e(|ual in the 

 aggregate to the total weight of the body they com- 

 [lose, yet it must not be forgotten th.it .ill the weighings 

 • iri' made in the same locality. 



.Suppose now that the constituents are weighed on a 

 sufficiently sensitive spring balance, and that some of 

 them are weighed near the equator and some near the 

 polar regions. If what we have said above about the 

 \ariation of weight with positions on the earth is true, 

 the sum of the separate weights will not equal the 

 weight of the compound or mixture. The total weight 

 of a body is admittedly not conserved, but depends 

 iijjon the conditions under which it is w-eighed. 



This variation canr.ot, however, be detected by an 

 ordinary balance. Tw'o weights that counterbalance 

 at one part of the earth will balance everywhere else; 

 in other words, though the pull of the earth on each 

 undergoes variation, it varies in the same proportion 

 for each; and, consequently, if they are equal anywhere, 

 the equality is universal. It is this peculiarity of an 

 ordinary balance which has led to the popular notion 



