The second point in prosecuting an inquiry into the pathology of any disease is to determine the train of poisoning when definite dosage, both in quantity and compound, is made use of. By feeding an animal with a compound only, the absorption through the gastro-intestinal canal could be studied; whereas by inoculating some of the compound—in suspension if it be insoluble, or in solution if it be soluble—into the subcutaneous or muscular tissue, the direct action of the body fluids on the compound may be studied; and, furthermore, its absorption by the membranes—that is, the cell membranes and the animal tissues—are determined. It is necessary to give at first a dose big enough to produce definite symptoms, and then to gradually decrease the dose to find the minimum amount producing symptoms within a reasonable amount of time. Inoculation experiments therefore give an answer to a number of these questions, and are the basis upon which further inquiry is conducted; they form a criterion from which it is possible to judge the effect of inhalation, and the same remarks which have been made with regard to inoculation refer to inhalation experiments. It is essential first of all, in the experimental animals, to subject them to rigorous enough conditions to obtain definite symptoms, and then, by varying the experiments, to study the amount, entrance, and general behaviour, of the poison, correlating the evidence so obtained from the definite knowledge already gained with the previous experiments.
It is hoped that this brief note on experimental evidence will assist in the elucidation of the foregoing experiments to those who are not conversant with the application of experimental evidence.
Further Experiments relating to Lead Poisoning amongst Painters.
—A series of further experiments were made, with particular reference to lead poisoning exhibited by painters; and as these experiments and their results could not have been undertaken without the previous knowledge gained of the pathology of lead poisoning due to the inhalation of particles of dust floating in the air, their discussion has been reserved until the previous section had been dealt with.
It has been supposed by some that surfaces painted with lead paint give off certain emanations containing the metal lead as an organic compound. As the incidence of lead poisoning amongst painters is exceedingly high, as far as any statistical evidence can be obtained (see [p. 48]), it would seem that the painter is peculiarly exposed to infection by lead dust; and if, in addition, organic compounds of lead were given off, he would be still more liable to lead poisoning.
Two methods of experiment were used:
1. The exposure of animals to the fumes given off from freshly painted surfaces, the paints used being compounded with white lead, lead sulphate, zinc sulphide, and zinc oxide.
Animals were exposed in a cage similar to that used in the inhalation experiments previously described, but, instead of blowing in the contaminated air, the cages were so arranged that boards freshly painted with the special paint under experiment were introduced into the cage daily, the animals remaining the whole time in the chamber. Special precautions were taken with regard to ventilation.
2. An animal was placed in a chamber, and the compound to be tested was heated electrically by means of a coil surrounding the glass tube in which the compound was placed. The current was regulated by means of resistances, so that the thermo-couple and galvanometer gave a constant reading of 59° C. Air was constantly passed through the tube over the heated substance and into the animal’s cage, which was efficiently ventilated. In this way any emanations which were given off from the normal room temperature or up to 59° C. were carried over into the animal’s cage, and there breathed. The apparatus was so arranged that the heating coil extended close to the point of delivery into the cage.
The result of these experiments showed that the animals confined in cages and exposed to freshly painted surfaces, where the paint used was white lead, zinc oxide, or lead sulphate, very soon showed signs of poisoning, and they became emaciated and suffered from recurrent attacks of salivation. The animals exposed in the cages in which air was passed over either white lead paste, zinc paste, or lead sulphate paste, showed no signs of illness, although kept in the cage and subjected to the inhalation of any fumes which might be given off for three months, spending the whole of the day in the cage, but being removed during the night to separate cages.