December 26, 1890.] 



SCIENCE. 



361 



other crystals and massive specimens found, but either very small 

 or insigniHcant in regard to quality or quantity. 



I expect this place to be to the mineralogist what its near 

 neighbor, Bergen Hill, world-famed, was fifteen years ago. 



E. W. Perky. 



Color- Changes in Toads. 

 In Mr. Poulton's ' Colors of Animals " he mentions (p. S3) that 

 ^'the common frog can change its tints to a considerable extent.'' 

 This recalls Pome experiments made in 1876 on common toads. 

 Toads kept on dark grounds or in dark pen became dark, and on 

 light grounds or in light pen became very much lighter in color. 

 The arrangement of colors was not changed, but the whole 

 seemed to grow lighter or darker. Red, blue, or other colors 

 seemed to have no effect except as to their value as light-absorbers. 

 ^Owing to the detailed notes being mislaid at this writing, it is 

 impossible to give details of time; but my remembrance is that 

 the change of shade took much longer than Mr. Poulton quotes 

 for the frog. The experiment is an easy and interesting one to 

 try. In fact, there is an immense field for young investigators in 

 the question of color, because of the ease with which apparatus 

 can be arranged and the number of species which as .vet have not 

 been tested. It is partly for this reason I record the above on 

 toads. Fbedeeio Gardner, Jun. 



Trinity College, Hartford, Conn., Dec. 17. 



The Cause of Rain. 



In your issue of March 7, 1 refer, on p. 161, in a letter with the 

 above title, to certain experiments which to my mind show con- 

 clusively that condensation takes place in saturated air by com- 

 pression, and not by expansion. 



As far as I am able to ascertain, the rain theories which my 

 letter brings me in opposition to are based upon the supposition 

 that the results of compressing and expanding saturated vapor or 

 steam, as mentioned in the mechanical theory of heat, may be 

 directly applied to saturated air. 



That these two so entirely different substances — the one a pure 

 gas, and the other a compound or mixture of various gases, and 

 both containing the same quantity of aqueous vapor for equal 

 temperatures, pressures, and volumes —should act in the same 

 way under compression and expansion seems to me hardly credi- 

 ble, and it vmy therefore be of interest to inquire whether any of 

 your readers can quote the results of experiments on saturated air 

 •which are contrary to those I have arrived at. 



Franz A. Velschow, C.E. 



Brooklyn, N.Y , Dec. 20. 



BOOK-REVIEWS. 

 A Treatise on Electro-Metallurgy. By Walter Q-. McMillan. 

 London Charles Griffin & Co. ; Philadelphia, J. B, Lippincott 

 Company. 12°. $3.50. 



This volume treats of the application of electrolysis to the plat- 

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 to others, as far as may be done through the medium of a book. 



The first chapter is mainly historical, briefly sketching the 



progress of the art, from the rude beginnings of its chemical side, 

 when it was discovered by the ancients that '•certain metals be- 

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 suitable solutions," dovvn to its latest developments, not omitting 

 the much-discussed Elmore process of producing seamless copper 

 tubes. In this chapter the scope of the art of electro metallurgy 

 is stated to be, (1) to obtain a coherent and removable deposit on 

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As a fair knowledge of chemistry and electricity is necessary 

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 useful tables. 



Outings at Odd Times. By Charles C. Abbott, M.D. New 

 York, Appleton. 16", $1.50. 



To those acquainted with Dr. Abbott's previous works, this 

 little volume needs no introduction. In spirit, if not in book- 

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 on occasional outings at odd times, in odd places, and under what 

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 there is much sound though unobtrusive philosophy in the book. 

 Dust and its Dangers. By Mitchell Prudden, M,D, New 

 York, Putnam. 16°, 75 cents, 



Dr, Prudden's interesting little volume, "The Story of the 

 Bacteria," is followed by another of equal interest and value. It 

 has been written " with the purpose of informing people, in sim- 

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 — especially consumption — by means of dust-laden air, and how 

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 the book will open the eyes of the people without needlessly 

 alarming them. Chief attention is given to the consideration of 

 the organic — bacterial and mould —constituents of dust-laden 

 air. The •' plate method " of biological analysis of air — i.e , five 

 minutes' exposure to the air of a film of nutrient gelatine, and 

 subsequent growth of colonies from the germs deposited — is 

 simple and reasonably accurate, and yields interesting results. 

 The relative numbers of bacteria in the air of the library of a pri- 

 vate house, of a large retail dry-goods store, and of a cross-street 

 in process of being cleaned, were 34, 199, and 3,810. Under ordi- 

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