592 THE POPULAR SCIENCE MONTHLY. 



for determining the composition of water, which is thus shown to con- 

 tain in one hundred parts 11*11 per cent of hydrogen and 88 '89 per 

 cent of oxygen. 



Another substance whose analysis can be very readily made by the 

 student is carbonate of magnesia. By igniting pure carbonate of mag- 

 nesia in a crucible (not of course the " magnesia alba " of the shops), 

 the proportions of carbonic acid and magnesia can be readily deter- 

 mined. Then, by burning magnesium ribbon, and weighing the prod- 

 uct, the student easily finds the relative weight of magnesium and 

 oxygen in the oxide. And, lastly, the proportion of carbon and oxygen 

 in carbonic dioxide is easily deduced from the burning of a weighed 

 amount of carbon. Here the result may be expressed either in per- 

 cents of oxide of magnesium and carbonic dioxide, or else in percents 

 of the elementary substances, carbon, magnesium, and oxygen. 



After making a few analyses like these, the student will be pre- 

 pared to comprehend the actual position of the science. All known 

 substances have been analyzed, and the results tabulated, so that it is 

 unnecessary to repeat the work except in special cases. 



The teacher is now prepared to take a very important step in the 

 development of the subject. If the molecule is simply a small par- 

 ticle of a substance in which the qualities of the substance inhere, then 

 it follows, of course, that the composition of the molecule is the same 

 as the composition of the substance. The percentage results of the 

 analysis of water, or of carbonate of magnesia, indicate the composition 

 of a molecule of water or a molecule of carbonate of magnesia. Thus, 

 11*11 per cent of every molecule of water consists of hydrogen, while 

 88*89 per cent consists of oxygen. Hence it follows that, in a chemical 

 process, the molecules must be divided, and these elementary parts of 

 molecules which analysis reveals are the atoms of chemistry. More- 

 over, as we know the weights of the molecules, both by physical and 

 chemical means, chemical analysis now gives us the weights of the 

 atoms. We have no time to dwell on the details of this reasoning, 

 but the general course to be followed will be evident, and it must be 

 enforced by numerous examples. 



Assuming that the student fully comprehends the distinction be- 

 tween molecules and atoms — that is, between the physically smallest 

 particles and the chemically smallest particles — he is prepared to mas- 

 ter the symbolical nomenclature of chemistry, with a very few words 

 of explanation. The initial letters of the Latin names are selected to 

 represent the atoms of the seventy known elementary substances, and 

 these letters stand for the definite atomic weights which are tabulated 

 in all chemical text-books. The symbols of the atoms are simply 

 grouped together to form the symbols of the molecules of the various 

 substances ; the number of atoms of each kind entering into the 

 oomposition of the molecule being indicated by a subscript numeral. 

 Lastly, in order to represent chemical processes, the symbols of the 



