290 BOTANICAL GAZETTE [Novenser | 
cal agents having the same number of molecules present in 
equal volumes of the various solutions. This is a decided step 
in advance, inasmuch as it enables investigators to compare the — 
properties of a molecule of any substance with those of any 
other molecule, a comparison obviously impossible under the 
old method of making up solutions of a certain per cent. by 
weight. The vital error in making a comparison between per 
centage solutions is readily seen when we recall that a I per 
cent. solution of formaldehyde contains over eight times a& 
many molecules per cc. as a I per cent. solution of mercuric 
chloride. — 
These equi-molecular solutions are made by dissolving @ 
many grams of the compound as there are units in its molecular — 
weight in 1000 grams of the solvent. Such a solution is termed 5 
a ‘‘normal solution” of that compound, and it is represented col . 
veniently by the formula ~. Diluted to half this strength we 
get , or one half normal solution, and so on. Solutions 
stronger than normal may be made by using double or quadruple 
the number of grams specified to the 1000% of the solvent, 
giving le and S: respectively, of the substance. 
I I : 
as show! 
Still more recent chemical and physical research h a 
m 
that in the case of very many substances in solution, 
the molecules of the dissolved substance are no longer f 
as such, but have become divided into two or more parts. : 
part-molecules have been termed ions. To illustrate, gas 
pose that 36.37 grams of pure HCI have been added to 
cc. of water; we know that the HCI is no longer all pi 
whole molecules. It fact, we have every reason to 
Th ey 
about 80 per cent. of it has become éonized into H at 
+ a 
the H ions being charged with positive electricity, 
having a corresponding negative charge. The 109°" cf 
stances capable of ionization are similarly ce ; 
