208 



SCIENCE. 



[Vol. YL, No. 136. 



reflected by magnesium carbonate, illuminated by 

 direct sunlight. Kepeated measurements of the rela- 

 tive intensities of corresponding portions of these 

 spectra throughout their whole length, and similar 

 comparisons of the spectrum of the magnesium car- 

 bonate with the direct spectrum of the source of 

 illumination, have furnished data from which the 

 character of the light sent us from the open sky can 

 be determined, and, in one sense, its color also. The 

 measurements show that the spectrum of the sky is of 

 the same character as that of white light, varying less 

 from the reflection spectrum of a perfectly colorless ob- 

 ject than do the spectra of such substances as white 

 paper, sulphate of calcium, carbonate of magnesium, 

 lamp-black, etc. Similar measurements were made of 

 the reflection spectra of LordKayleigh's ' blue cloud,' 

 formed by the precipitation of sulphur by hydrochloric 

 acid in a solution of hyposulphite of sodium, and of 

 thin films of antimony oxide. It was found that the 

 same is true of the light reflected by these substances. 

 The blue color of the sky and of other opalescent me- 

 dia is, according to these results, not due to an excess 

 of the more refrangible rays in the light reflected by 

 them, but is of subjective character. This view the 

 author has maintained in a previous paper, in which 

 it was pointed out that a well-known peculiarity of 

 the eye, its rapidly increasing sensitiveness to violet, 

 with decrease of intensity of illumination, is sufficient 

 to account for the appearance of the sky, and of 

 many other objects, without having recourse to the 

 hypothesis of selective reflection. The object of the 

 present paper is the presentation of experimental 

 evidence bearing upon this question. It is to be re- 

 gretted that Prof. S. P. Langley had left town before 

 this paper was read. A number of those present 

 called attention to the disagreement of the results of 

 Professor Nichols with those obtained by Professor 

 Langley. 



Professor Nichols's paper was appropriately fol- 

 lowed by that of Prof. C. K. Wead, who exhibited a 

 combined spectro-photometer and ophthalmospectro- 

 scope. This instrument, made by the Geneva society, 

 is intended to combine with the least possible dupli- 

 cation of parts, several of the best instruments for 

 the study of spectra. It gives the measure of the 

 relative brightness of the spectra: 1°, by the width 

 of slits or by smoked glasses; 2°, by Nicol prisms 

 outside the collimator; 3°, by combination of the 

 two. Further, it allows: 1°, the mixture of any two 

 spectral colors in any relative intensity, and their 

 comparison with an intermediate spectral color by 

 'Bonder's coupled slits;' 2°, the addition of either 

 the simple or the mixed color of a measured quantity 

 of white light (Glan) ; 3°, the comparison of the sim- 

 ple or mixed colors of the spectrum with the light 

 from a colored body. 



In a paper on weather changes of long period, Mr. 

 H. Helm Clayton of Ann Arbor cited evidence that 

 there are at times slow progressive movements of 

 barometric change, and of temperature from west to 

 east. Mr. Clayton also made an attempt to show a 

 certain periodicity in the character of the weather of 

 the United States during the last year, and claimed to 



be able to make predictions based on this periodicity 

 for a month in advance. The paper excited consid- 

 erable adverse criticism. 



Two papers by Dr. J. W. Moore of Easton, Penn., 

 were devoted to an explanation of apparatus for class- 

 room demonstration of electrodynamic phenomena. 

 A paper read by Prof. H. W. Eaton of Louisville, 

 on the relation of vanishing and permanent mag- 

 netism, contained results of an investigation which 

 he had undertaken at the suggestion of Wiedemann. 



Prof. C. J. Reed of Burlington, lo., exhibited a 

 piece of apparatus for classroom demonstration of the 

 laws of falling bodies. 



CHEMISTRY IN THE SERVICE OF 

 PUBLIC HEALTH.-^ 



In the study of hygiene from the chemical side, we 

 are obliged to consider not only the normal condi- 

 tions of the earth and atmosphere, but the changes 

 which are brought about by the crowding together of 

 individuals on account of the pursuit of manufactur- 

 ing industries. 



In the service of sanitary science, chemistry has an 

 educational office to fill. The public has very little 

 conception of what the capabilities and limitations 

 of chemistry are. It is hard to make a person be- 

 lieve that water to be analyzed must be brought in 

 a clean vessel, and that the chemist cannot distin- 

 guish between the impurities of the water and those 

 of the jug. It is almost impossible for the chemi- 

 cally uneducated public to understand that when 

 chemical action takes place, the properties of the 

 substances concerned are not carried into the prod- 

 uct ; that because vitriol is used in glucose factories 

 the product does not contain the acid ; and the use 

 of aquafortis in making oleomargarine, is equally 

 startling. 



There must needs be reformers and philanthro- 

 pists, but many of these are extremists; and nowhere, 

 more than in sanitary matters, is a little knowledge a 

 dangerous thing. At one time all the evils were at- 

 tributable to microbes, and at another to sewer-gas. 

 Microbes may be left to the biologists, and possibly 

 sewer-gas as well, since chemists have failed to dis- 

 cover any substances in the gas which could produce 

 the well-known ill effects. In the matter of food 

 adulteration, the origin of the terror is often obvi- 

 ous; thus, that tea is said to be adulterated with 

 prussic acid, arose from the use of Prussian blue in 

 the facing. Chemists are periodically obliged to dis- 

 tinguish between adulterations which are merely 

 falsifications and those which are harmful, and it 

 must be remembered that even the purest commer- 

 cial products contain small amounts of foreign sub- 

 stances. 



It is, perhaps, not altogether to our credit that we 

 so often need the spur of extravagance to lead us to 



1 Abstract of an address delivered before the section of chem- 

 istry of the American association for the advancement of science, 

 at Ann Arbor, Aug. 26, by Prof. W. R. Nichols, of the Massachu- 

 setts institute of technology, Boston, vice-president of the section. 



