96 PRINCIPLES OF CHEMISTRY. 



and also that it enters into union with other elements in the smallest 

 proportions. Hence in establishing the above-mentioned system of 

 numbers, one part by weight of hydrogen is taken as the standard of 

 reference, which offers the advantage that none of the figures is less 

 than unity. Let us illustrate by a few examples: One part of 

 hydrogen unites with 35.18 parts of chlorine, 35.18 parts of chlorine 

 unite with 38.82 parts of potassium, 2 X 38.82 parts of potassium 

 unite with 15.88 parts of oxygen. But 2x1 parts of hydrogen also 

 unite with 15.88 parts of oxygen. Also 15.88 parts of oxygen unite 

 with 64.91 parts of zinc, and 64.91 parts of zinc unite with 2 X 35.18 

 parts of chlorine. Thus we see that the figures in these instances are 

 reciprocally related, and the same is true for the figures assigned to 

 all the other elements. This system of figures is called combining 

 weights, and has a deep significance, which will appear clearer when 

 the atomic theory is discussed. 



Atomic theory. One of the objects of men of science, besides 

 experimenting and observing facts and laws, is to determine causes^ 

 that is, to furnish an answer to the question, Why do things take 

 place as they do? If the senses, even when fortified with delicate 

 instruments, are not sufficiently refined to perceive the causes, 

 the philosopher, with the aid of his reasoning faculties, tries to 

 imagine a cause which will account in the most satisfactory manner 

 for what he observes. Such an imagined cause is called an hypothe- 

 sis or theory. For example, to account for the uniform behavior of 

 all gases as summed up in the Laws of Boyle, Charles, and Avogadro, 

 it was imagined that the nature of gases is that they are made up of 

 exceedingly minute particles in motion, acting as elastic bits of 

 matter and with practically no cohesion between them. This is 

 known as the kinetic-molecular theory of gases, and from it all the 

 behaviors of gases can be deduced mathematically. We may never 

 be able to actually see a gas particle, nevertheless we are willing to 

 accept that it exists as long as the hypothesis accounts satisfactorily 

 for what is observed. 



There must be some reason for the chemical behavior of the 

 elements as set forth in the Laws of Definite and Multiple Propor- 

 tions and the fact that the elements unite in proportions represented 

 in the system of figures called combining weights. This cause must 

 lie in the constitution or physical make-up of matter. Reflection 

 upon the constitution of matter is not peculiar to modern science, for 

 the ancients had their conceptions, and some (Democritus, Lucretius) 



