ß TESTING COSMETICS CONTAINING RADIOISOTOPES 279 as to the nature of the animals to be used. Finally, it was decided to select an animal which is widely used for' the testing of cosmetics and other' substances for the pres- ence of primary irritants. This animal is the American chinchilla rabbit, which is a recognized breed produced by various breeders of laboratory animals in the country. The American chinchilla rabbit is the best test animal for testing the presence of primary irritants in all types of materials by intradermal injection. Its skin is very com- parable to human skin for reaction to cosmetics and penetration by osmosis. Contrarily to the angora or to the albino rabbit, it is not hypersensitive to any cosmetics or chemicals, and, therefore, the re- sults obtained on one animal are always reproducible on others. Prior to the test all animals were freshly shaven and the cream con- taining the labeled spermaceti was applied on the skin of the animals by gentle massage, thus reproduc- ing normal conditions of use of the cosmetics on the face. The dura- tion of the massage was 1, 2, 5, 10, 20, and 30 minutes. Other applications of the cream were made by massaging the skin for half an hour and leaving the cream on the skin for another 1/2 hour, 1 hour, 21/=hours, 41/= hours, and 61/2hours. Consequently, pene- tration through the skin was studied after periods of time which varied fi'om 1 minute to 7 hours. During the long applications, the animals were held in a special device where they could eat and drink leisurely, but could not remove the cream by friction or otherwise on any part of the holder. Counts were made at the end of each period of application. First, immediately after the excess of cream had been thoroughly removed with sterile paper tissues and also after the skin had been thoroughly cleaned with a standard mild soap solution. All counts were made by means of an extremely thin end window G. M. counter. One of the most important in- struments in work dealing with radioisotopes is the Geiger counter tube. In its simplest form, it consists of a tube, very similar to a radio tube, into which have been sealed two metallic terminals. One terminal runs along the side of the tube and the other through the center. The center wire is charged positively in respect to the side terminal, which is charged negatively. These terminals are attached to a high voltage source and filled with a suitable gas. When the Geiger counter is not in operation, i.e., no radioactivity is exposed to the tube, no electric current will flow across the ter- minals. But when a high energy particle enters the tube, such as an alpha or beta particle, ionization of the gas occurs and a pulse of current flows. This current is then amplified and can operate a mechan- ical counting device. Using this. principle, this simple instrument is able to detect the presence of individual atomic particles.

:280 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß At one end of the Geiger tube there is constructed a window, usu- ally made of mica, through which the radioactive particles can enter and be recorded. The thickness of this end window is expressed in milli- grams per square centimeter of material. When counting weak beta radiation, such as we find emanating from the sample of Carbon 14, it is necessary to con- struct a mica window of extreme thinness, so that a good portion of the beta particles will enter. The thickness of this end window should, preferably, be less than 1 milligram per square centimeter. The thicker the window, the greater the stopping power toward the weak beta particles and the lower is the counting efficiency of the tube for this particular Carbon 14 radiation. RESULTS OF THE COUNTS High counts were obtained on the cream before application to the animal and on the skin of the animal itself. However, counts made immediately after the excess of cream had been removed with paper tissues were negative. No counts were obtained after any of the applications. No radiation was, of course, observed after the test skin surface had been cleaned with a standard mild soap solution. STUDY OF BIoPSIES Skin sections, measuring one inch in diameter, were cut from one of the animals. The sections were mounted in white paraffin and quick frozen at a temperature of -195 ø C. in liquid nitrogen. The specimens were then mounted in a microtome and several longitudinal sections of 10 micron thicknesses were cut. The sections were then placed separately in a Q-gas cham- ber, and examined for the extent of radioactivity. There was no significant radiation emitted from the test surface. The counts were made of a portion of the skin tissue, and again no evidence of penetration of the tagged part of the cream into the skin was found. CONCLUSION This series of experiments in- dicate only one thing: that sper- maceti as part of a cold cream does not penetrate the skin under normal conditions of use and also after periods of massage and application as long as 7 hours. For this reason these tests should be con- sidered only as an introduction to the use of radioisotopes in cos- metic chemistry, and as an in- dication of the results that might be obtained by following the same technique on other types of cos- metics. This paper should be re- garded as a preliminary investiga- tion whose results will probably encourage further studies which will be of interest to cosmetic chemists and dermatologists. It should be borne in mind, however, that the use of radioisotopes is expensive, deli- cate, and that the interpretation of the results can be done only by technicians whose experience is based on the study of radioisotopes.