CHEMISTRY. 



143 



nate method affords a quick and ready means 

 for the approximate valuation of indigoes, but 

 the results obtained are sometimes too high. 

 The method of precipitation with sodium 

 chloride and titration with potassium perman- 

 ganate gives results which, for all practical 

 purposes, are trustworthy. 



The accuracy of the soda-lime process for 

 determining nitrogen having been questioned, 

 W. (). Atwater and C. D. Woods have given 

 attention to the methods of manipulation and 

 the sources of error and ways of avoiding them, 

 and have been convinced that when rightly 

 managed it gives excellent results. At the 

 same time they decline to say that they regard 

 the soda-lime method as entirely reliable, even 

 for protein compounds, unless all needed pre- 

 cautions are observed. 



To detect and measure magnetic susceptibil- 

 ity in substances which show no evidence of 

 magnetism under the usual processes, Mr. T. B. 

 Warren places a weight of the substance experi- 

 mented upon in the pan of a chemical balance 

 which is adjusted to the magnetic meridian; 

 equilibrium having been made, a magnet is 

 placed directly under the scale-pan, when, if the 

 substance is paramagnetic or positive, the pan 

 will be drawn down. The weights that have to 

 be added to restore equilibrium give the meas- 

 ure of the susceptibility of the substance in hand. 

 Diamagnetic or negative substances are also 

 attracted under the same condition, instead of 

 being repelled, as might be supposed ; and the 

 author infers from this that magnetic repulsion, 

 in a positive sense, does not exist. To measure 

 magnetic permeability, a plate of the metal or 

 stratum of the liquid is inserted between the 

 magnet and some iron-filings. When the plate 

 is removed, the magnet is attracted to within 

 a rixed distance of the filings, and the weight 

 required to produce equilibrium is noted, the 

 plate is then inserted, and the diminished 

 attraction is again noted. The difference in 

 weight is due to the arrest of magnetic influ- 

 ence by the interposed layer. 



Sulphuric acid and naphthalamine hydro- 

 chloride have been found by C. E. Howard to 

 be most delicate and satisfactory reagents for 

 detecting the presence of nitrogenous and 

 chloride impurities in drinking-water. Water 

 bnt slightly tainted with nitrous acid only 

 gives a very faint pink on application of these 

 In proportion as the contamination is 

 greater, the coloration is more intense, until a 

 deep carmine is produced. The reagents for 

 chlorides, the presence of considerable quanti- 

 ties of which may indicate contamination by 

 animal excreta, nre nitric acid and silver 

 nitrate. They produce in water containing 

 chlorides a white precipitate of silver chloride, 

 the exhibition of which rises from a mere opal- 

 escence when the quantity of chloride is slight 

 to a distinct deposit when the contamination 

 is considerable. 



Chemistry of the Arts. W. X. Hartley has 

 shown that light may effect changes in organic 



coloring substances in other ways than by 

 promoting oxidation or reduction, thus: The 

 color of an organic substance is an effect of 

 its highly complex structure, notwithstanding 

 the fact that its composition may be simple 

 enough. It may consist, for instance, of but 

 three or four elements carbon, hydrogen, and 

 oxygen, with, perhaps, nitrogen but the num- 

 ber of atoms necessary to produce the smallest 

 particle or molecule of color is large ; and every 

 color depends upon the way in which the atoms 

 are arranged in the molecule. The shifting of 

 a single atom will cause a brilliant color to be- 

 come colorless. The effect of light on >u-h 

 substances is variable; sometimes the change 

 induced is oxidation ; it is sometimes a molecu- 

 lar change, or the rearrangement of the atoms 

 in the molecule. Light may also be capable 

 of resolving a complex substance into two or 

 more simpler substances. The color of a sub- 

 stance depends upon the rate of vibration of 

 its molecules. The more brilliant the light 

 the more ample are the vibrations. It is easy, 

 then, to understand how a light of great brill- 

 iancy may throw a colored molecule into such 

 a state of intense vibration that the molecule 

 will fall to pieces. The complex and unstable 

 compound is resolved into two or more simple 

 and colorless bodies. Unstable colors are also 

 liable to be changed by oxygen, which is never 

 excluded from framed pictures ; moisture, 

 which is used in the mounting of pictures, and 

 is in the air ; and acidity, which exists to a 

 greater or less extent in all towns where coal 

 is burned, and which is sometimes a property 

 of the paper on which drawings are made. All 

 preparations of lead are sensitive to impurities 

 of the air, and should never be used in works 

 of art; and of mercury, only pure cinnabar or 

 vermillion. Acidity may be partly remedied by 

 washing the paper in a slightly alkaline solu- 

 tion, or by using weak borax- water in applying 

 the pigments. Of the various colors, the 

 yellows and crimson are most affected by sun- 

 light, and blue and gray tints by an impure 

 atmosphere. The difference in the effect of 

 direct sunlight and diffused daylight upon 

 colors is very great. In the latter the prevail- 

 ing rays are the yellow ones, while the violet and 

 ultra-violet, rays, which are so active in direct 

 sunlight, are absent. Diffused light sufficient 

 for the exhibition of pictures is forty times 

 weaker than average direct sunlight, or four 

 hundred times weaker than that of summer. 



In a paper read at the British Association 

 on ''The Action of Light on Water-colors. 1 ' 

 Dr. Arthur Richardson named cadmium yel- 

 low, cadmium orange, king's yellow, and 

 indigo, a< colors which hleach by oxidation 

 under the combined influence of light, air. and 

 moisture, but are permanent in an atmosphere 

 of carbon dioxide or in dry air. A second 

 group of colors on which light exerts a reduc- 

 ing action, which is independent of air, and in 

 some cases takes place in the absence of moist- 

 ure, includes Prussian blue, vermillion, lakes. 



