Laws of Excitation by Ultraviolet Rays.

(Victor Henri, Archives of Physical Sciences, Nos. 1 and 2, 1918).

The article placed in the title is a brief monograph, summarizing a whole series of works by the author and his students. This monograph once more shows how many valuable and often unexpected results have been yielded in recent years by the application of physico-chemical methods to the study of problems of biology and physiology. First of all, it should be noted that the boundaries between physics and chemistry have been definitively erased. If formerly one could divide phenomena into physical and chemical ones, the former being defined as those in which the composition of the substance does not change, then now such phenomena as radioactivity, photochemical reactions, the photoelectric effect, capillarity phenomena, and a whole series of processes of so-called physical chemistry undermine the distinctness and categorical character of these distinctions.

definitions. There are no boundaries. The aforementioned fields have expanded so much, and for them there has accumulated such a rich body of factual and theoretical material, important both for physics and for its applications, that it has become necessary to allot them considerable space in proportion to their contemporary significance. It may be asserted that physics and chemistry have ceased to be what they were ten or fifteen years ago. Secondly, the natural sciences have passed over to quantitative measurements and have accumulated so broad a stock of facts that it has become possible, in many cases, to embrace these fields of knowledge by mathematical relations, making it possible to establish a quantitative connection between the various elements of the phenomenon under study.

The works of V. Henri considered in the article have been carried out at different times and concern the application of physicochemical methods to biological questions. The author set himself the goal of investigating excitation by a definite part of the spectrum, namely the ultraviolet. Ultraviolet rays were chosen because the author had found a connection between the coefficient of absorption of ultraviolet light and various groups of atoms. This made it possible, in studying phenomena of irritation, to establish which groups are acted upon by the irritating rays. Small (0.5–1 mm) free-living animals (Cyclops) were chosen as the objects for irritation.

First of all, the threshold of irritation was determined, and then the dependence of this threshold on the intensity of the irritating energy was established. It turns out that the energy corresponding to the threshold passes through a minimum. The optimum duration of irritation proved to be equal to one second. For durations greater than a second, the dependence fully obeyed Lazarev’s law:

\[ SO = \frac{B}{\alpha_1 k \left(1 - \frac{B}{2}\right)} + \frac{Bc_2}{2\alpha_1 k \left(1 - \frac{B}{2}\right)}\, t . \]

For short times of illumination corresponding to the threshold, it becomes necessary to introduce new factors.

If the speed of the animal’s reaction to this irritation is taken as a measure of irritation, it turns out that the irritation of Cyclops by ultraviolet rays obeys a psychophysical law. This is connected with the observations that must be adopted for actions of irritation of the eyes, nerves, and muscles. A study of irritation under intermittent illuminations showed that, for illuminations separated by a considerable interval, intermittent illumination acts more strongly than continuous illumination. The author puts forward a hypothesis of photochemical induction and connects it with those studies of the action of the ultraviolet spectrum on organic colloids in the presence of electrolytes which were begun by the author in Paris and are being continued in Moscow.

A study of the influence of temperature (6°–27°) gave the rather unexpected result that the irritability of the animal does not change with a change in temperature.

Finally, the author discovers an extremely important circumstance—a complete and definite parallelism between the curve of irritation and the curve of absorption in the region of wavelengths from 2550 to 3000, the dependence established by Lazarev (the photochemical law) for the fading of dyes.

B. V. Ilyin.