July 4, 1902.] 



SCIENCE. 



33 



(apparent) upper edge of the cloud. The lat- 

 ter moved in an easterly direction, away from 

 the sun, and in four or five minutes the colors 

 had faded away. A few minutes later another 

 patch of the same kind of cloud, also drifting 

 east, occupied ahout the same position as that 

 taken by the first cloud at the time it became 

 iridescent, and this second cloud, in its turn, 

 showed faint rainbow coloring. This phenom- 

 enon was repeated three times, and in no case 

 did the iridescence last more than four or five 

 minutes. The colors were brightest in the 

 second cloud. There were a good many 

 patches of cirro-stratus in different portions of 

 the sky at the time, and several of them 

 showed waves. Light local showers occurred 

 during the evening or night following. 



Studies of iridescent clouds have been made 

 in Europe by Ekholm, Schips, Mohn, McCon- 

 nel, Hildebrandsson, Kassner and others. A 

 useful article in this subject, by Arendt, will 

 be found in Das Wetter for 1897, pp. 217-224, 

 and 244^252. In the Jahrbuch fur Photog- 

 raphie und EeproductionstecJuiih for 1900, in 

 a brief article on the same subject, by Kass- 

 ner, there are some half-tones of iridescent 

 clouds. The views do not, of course, repro- 

 duce the colors. E. DeC. Ward. 



Harvard University. 



physics and the study of medicine. 

 To THE Editor op Science : Dr. Trow- 

 bridge, in his paper on 'The Importance of a 

 Laboratory Course in Physics in the Study of 

 Medicine,' Science^ May 30, 1902, mentions 

 the Johns Hopkins as one of the medical 

 schools that do not ofl^er a laboratory course 

 in physics. His statement is correct, but the 

 inference that might be drawn from it, 

 namely, that the Johns Hopkins does not con- 

 sider such a course an important part of 



the preparation for medicine, is entirely incor- 

 rect. Those who are familiar with the re- 

 quirements for medical study in this country 

 are aware, of course, that from its foundation 

 in 1893 the Johns Hopkins has required from 

 each of its entering students certificates not 

 only of a college course in physics, but of a 

 laboratory course as well. If, as frequently 

 happens, the student has not been able to get 

 a laboratory course in the college from which 

 he comes, he is entered as conditioned in 

 laboratory physics and is obliged to absolve 

 this condition during his first medical year by 

 attendance upon a course provided for such 

 cases. W. H. Howell. 



Johns Hopkins Medical School. 



SHORTER ARTICLES. 



ON A method in HYGEOMETRY. 



During the course of my work on the dif- 

 fusion of nuclei in hydrocarbon vapors, 1 

 noticed that on certain days the experiments 

 were apt to break down; the column of air 

 within the tower-like receiver, instead of show- 

 ing on exhaustion the sharp plane of demark- 

 ation between the nucleated air below and the 

 pure air above, was liable to condense as a 

 whole, almost explosively. This occurred at 

 a definite pressure and after condensation had 

 already begun in the nucleated region. Sus- 

 pecting that the discrepancy might be due to 

 the hygrometric state of the atmosphere, I 

 made the following tests which bear out this 

 surmise. The first column shows the pressure 

 decrement on exhaustion, the second the effect 

 produced on the nucleated atmospheric air in 

 the dry receiver. In the second and third 

 parts of the table, the results of artificially 

 moistening and of drying the air are at once 

 apparent. 



