156 REPORTS ON THE STATE OF SCIENCE.—1918. 
large doses (12 grammes daily) that the excess of it produces un- 
desirable effects. Such a difficulty clearly shows that the medicine 
has been administered in an unsuitable form. 
The chief advances in chemistry during the past thirty years have 
been due to the development Of the theory that chemical reactions 
occur between the ions or ultimate portions into which compounds 
are decomposable. Hence,an element in which the body is deficient 
must eventually be reduced either to its ionic state or to a form 
of combination which is equivalent to it. 
In many cases, substances in a colloidal state react with an 
activity which is otherwise unobtainable. This may, in part, be due 
to the fact that when complete dissociation into ions is required the 
solutions must usually be exceedingly dilute, whereas colloida! solu- 
tions may be much stronger. Hence, the use of colloids has proved 
invaluable as a means of making good the deficiency in certain 
elements from which some tissues suffer. 
Diseases which require selective remedies (group c) are chiefly _ 
those due to parasites, including the bacilli, micrococci, and other 
“germs” which produce certain poisons or toxins. 
A substance which exerts either a toxic or a healing action on the 
body usually shows a greater affinity for one set of tissues than for 
others. Some substances have a greater action on bactaria and other 
parasites than on the host, and vice versd. In other words, “heir 
action is selective, and an ideal medicine would be one which is so 
. powerfully selective as to be absolutely fatel to the parasites it is 
desired to destroy and yet wholly harmless to the patient. 
Ehrlich showed that a toxin is capable of producing disease only 
in those persons whose body-cells contain substances capable of 
entering into chemical combination with the particular toxin. 
The anti-toxins and bacteriolysins appear to equally definite in 
composition. Ehrlich found that an aniline dye, to which he gave 
the name of trypan red, is highly toxic with respect to trypanosomic 
parasites in blood but relatively harmless to the host. 
According to Metchnikoff’s popular theory of phagocytosis, the 
wandering corpuscles or leucocytes in the blood attack the 
bacteria or other parasites, but this theory has receded in conse- 
quence of the discovery by G. F. Nuttall, who, when working in 
Fliigge’s laboratory in 1886, found that anthrax and other bacteria 
died rapidly in fresh blood serum without the interposition of the 
leucocytes, thus showing that the serum itself contained the 
(chemical) substances which brought about their destruction. This 
is to some extent incompatible with Erlich’s endeavours to seek a 
substance which is parasito-tropic without being organo-tropic, 
because a drug injected into the blood stream—either directly or 
intramuscularly—does not usually exhibit any action until it has 
been taken up by the serum. Hence the organo-tropic (as distinct 
’ from the organo-toxic) properties of a drug are of great importance. 
So far, no substances have been found which are highly parasito- 
tropic and wholly devoid of organo-tropic properties, though many 
colloidal sols are so feebly organo-tropic as to be devoid of danger in 
any ordinary doses. In the case of elementary colloids such as 
mercury or iodine, any toxic symptoms and ill effects may be re- 
