Uniform and Constant Saturation. 285 



defiant gas ; and C 2 H 2 , acetylene, — just what the theory de- 

 mands. Passing on to the hydrocarbons of triple condensation, 

 theory indicates the existence of C 3 H 8 , C 3 H 6 , C 3 H 4 , and C 3 H 2 , 

 and denies the possibility of any other forms. As a matter of 

 fact, chemists know C 3 H 8 , C 3 H 6 , and C 3 H 4 — C 3 H 2 being yet 

 undiscovered; but besides these, no other hydrocarbon contain- 

 ing only C 3 has been found. Of the C 4 series, theory requires 



C 4 H 10 , C 4 H 8 , C 4 H 6 , C 4 H 4 , C 4 H 2 ; - 



and four of them are known. In short, we know an immense 

 multitude of hydrocarbons of all degrees of condensation, con- 

 taining in the standard volume C l up to C 30 and more; and 

 there is not a single case which is in opposition to the theory. 

 Whatever the number of atoms of carbon which may be piled 

 up together, the saturating power of the carbon is invariably equal 

 to four atoms of hydrogen. 



Nothing is calculated to give a better idea of the scope of the 

 modern doctrine of uniform saturation than a survey of the 

 different phases through which the atomic theory has passed. 



As originally propounded by Dalton, and as understood for a 

 number of years and maintained by old stagers even at the pre- 

 sent day, the theory was just this : — matter is made up of indivi- 

 sible atoms which vary in weight according to the kind of matter. 

 The elements are composed of these atoms. When atoms of differ- 

 ent kinds link themselves together, there result groups of atoms. 

 Compounds are composed of these groups of atoms. With the 

 nature of the grouping, whether, for example, it should necessarily 

 be a group of two, three, or a dozen atoms that constitute a 

 group, the original atomic theory had no concern. All that was 

 contended for was that the group should not be a very compli- 

 cated one. The basis of experiment which was represented by 

 this crude state of the theory was this fact. If we take the ana- 

 lysis of any one of the common inorganic compounds, and instead 

 of writing the quantity of each element contained in a hundred 

 parts, divide the quantity of each element by a certain factor 

 varying for each element, we obtain a comparatively simple 

 numerical expression, or at any rate an expression which is 

 easily converted into a very simple one. The factor then became 

 the atomic weight : the simple numerical expression which, when 

 reduced, was often as simple as 1:1, or 1:2, or 2:3, &c, 

 expressed the numbers of the different atoms composing the 

 group. When the atomic theory came to be applied to the com- 

 pounds belonging to organic chemistry, it was found that the 

 numerical expression arrived at by this process was anything but 

 simple, being now and then quite as complicated as the per- 

 centage statement of the analysis. In presence of this difficulty, 



