i6 



NATURE 



\Nov. 7, 1889 



bines readily with the oxygen of the air at the ordinary 

 temperature, and he claims that the iron so set free is 

 allotropic ; but Joule did much more than this. Magnus 

 had shown (1851) that the thermo-electric properties of 

 hard and soft steel and iron differ. Joule, in a paper on 

 some thermo-electric properties of solids, incidentally 

 shows that the generation of a thermo-electric current 

 affords a method of ascertaining the degree of carburiza- 

 tion of iron, and he appeals to the " thermo-electricity of 

 iron in different states " as presenting a "fresh illustration 

 of the extraordinary physical changes produced in iron by 

 its conversion into steel," and he adds the expression of 

 the belief " that the excellence of the latter metal might 

 be tested by ascertaining the amount of change in 

 thermo-electric condition which can be produced by the 

 process of hardening." ^ It is by a thermo-electric method 

 that the views as to the existence of iron in allotropic 

 forms has been confirmed. Jullien seems to have inclined 

 to the view that iron is allotropic in his "' Theorie de la 

 Trempe," ^ published in 1 865, but he cannot be said to have 

 added much to our knowledge, although he certainly 

 directed attention to the importance of hardening and 

 tempering steel. 



The next step v/as made in Russia, in 1868. Chernoff, 

 who has found an admirable exponent to English readers 

 in Mr. W. Anderson, President of Section G, showed that 

 steel could not be hardened by rapid cooling until it had 

 been heated to a definite temperature — to a degree of 

 redness which he called a. Then in 1873, Prof Tait^ 

 used this expression in a Rede Lecture delivered at 

 Cambridge : " It seems as if iron becomes, as it were, a 

 different metal on being raised above a certain tempera- 

 ture ; this may possibly have some connection with the 

 ferricum and ferrosum of the chemists." He also 

 published his now well-known " first approximation to a 

 thermo-electric diagram," which is of great interest in 

 view of recent work. At about this time those specially 

 interested in this question remembered that Gore^ had 

 shown that a curious molecular change could be produced 

 by heating an iron wire, which sustains a momentary 

 elongation on cooling. Barrett repeated Gore's experi- 

 ment, and discovered that as an iron wire cools down 

 it suddenly glows, a phenomenon to which he gave the 

 name recalescence^ and these investigations have been 

 pursued and developed in other directions by many skil- 

 ful experimenters.''" In 1879, Wrightson** called attention 

 to the abnormal expansion of carburized iron at high 

 temperatures. 



The next point of special importance seems to me to 

 be that recorded by Barus, who, by a thermo-electric 

 method, showed, in an elaborate paper published in 1879,^ 

 that " the hardness of steel does not increase continuously 

 with its temperature at the moment of sudden cooling, but 

 at a point lying in the dark-red heat the glass-hard state " 

 may suddenly be attained by rapid coohng. I shall have 

 again to refer to the remarkable series of papers published 

 by Barus and Strouhal,^ embodying the results of laborious 



' Phil. Trans., cxlix., 1859, P P'- 



2 " Annexe au traite de la Metallurgie du Fer," 1865. 

 ^ Nature, viii., 1873, pp. 86, 122; and Trans. Roy. Soc. Edin., xxvii., 

 1873, p. 125. 



4 Proc Roy. Soc , xvii., 1869, p. 260. 



5 G. Forbes, Proc. Roy. Soc. Edin., viii., 1874, 363 ; Norris, Proc. Roy. 

 Soc, xxvi., 1B77, 127 ; Tomlinson, Phil. Mag., xxiv., 1887, 256; xxv.,pp.45, 

 103, and 372 ; xxvi. p. 18 ; Newall, Phil, Mag., xxiv., 1887, 435 ; xxv , 1888,, 

 p. 510. 



6 Journ. Iron and Steel Inst., No. ii. 1879 ; No. i. 1880. 

 '' Barus, Phil. Mag . viii., 1879. p. 341. 



** " Hardness (Temper), its Electrical and other Characteristics," 

 Barus,/"////. Mag., viii p. 341, 1879; ^'cd. Ann., vii. p. 383, 1S79 ; 

 Strouhal and Barus, VVied. Ann., xi. p. 930. 1880; ibid., xx. p. 525, 1883. 

 ''Hardness and Magneiization," IVied. Ann., xx. pp. 537, 662. 1883. 

 "Density and (Internal) Structure of Hard Steel and of Quenched Glass," 

 Barus and Strouhal, American Journ., xxxi. p. 386, 1886; ibid., p. 439 ; 

 ibid., xxxi. p. 181, 1886. "Temper and Chemical Composition," Ant. 

 Joiim., xxxii. p. 276, 1886 "Temper and Viscosity," Am. Journ., xxxii. 

 p. 444, 1886 ; tbid., xxxiii. p. 20, 1887 ; Barus, ibid., xxxiv. p. i, 1887 ; ibid., 

 xxxiv. p. 175, 1887. These paper.;, systematically discussed and enlarged, 

 are embodied with new matter in the Bulletins of the United States Geo- 

 logicaf Survey, viz. : — Bull., No. 14, pp. 1-226, 1885; Bull, No. 27, pp. 

 ^0-61, 1886; Bull., No. 35, pp. ii-6o, i836 ; Bull., N0.42, pp. 98-131, 1887. 



investigations, to which, in the Hmited space of this lec- 

 ture, I can do but scanty justice ; and finally, within the 

 last few years, Pionchon ^ showed that at a temperature of 

 700° the specific heat of iron is altogether exceptional, and 

 Le Chatelier - has detected that at the same temperature a 

 change occurs in the curve representing the electromotive 

 force of iron — both experimenters concluding that they had 

 obtained evidence of the passage of iron into an allotropic 

 state. 



Osmond,^ in France, then made the observations of 

 Gore and Barrett the starting-point of a fresh inquiry, 

 which will now be considered at some length, -as 

 Osmond has arrived at conclusions of much interest and 

 importance. 



{To be conttmted.) 



ON A NEW APPLICA PI ON OF PHO TOGRAPHY 

 TO THE DEMONSTRATION OF CERTAIN 

 PHYSIOLOGICAL PROCESSES IN PLANTS. 



TWrR. WALTER GARDINER, Lecturer on Botany in 

 -'■*-•■ the University of Cambridge, who delivered the 

 evening address at Newcastle on " How Plants maintain 

 themselves in the Struggle for Existence," has discovered 

 a new method of printing photographic negatives, employ- 

 ing living leaves in place of sensitive paper. Mr. Gardiner 

 read a paper on the subject before the British Association. 

 Before dealing with the immediate subject of his paper, the 

 author described how prints may be obtained from Proto- 

 cocci, or the free-swimming swarm-spores of many green 

 Algae. It is possible to take advantage of their sensitive- 

 ness to light. Into one end of a watertight box, a thin 

 glass plate is securely fitted. The negative to be printed 

 is then placed next the glass, film side nearest. The box 

 is filled with water containing a fairly large quantity of 

 swarm-spores. The lid is shut down, and the whole is 

 exposed to diffused light. In the case of a strong and 

 well-developed negative, the swarm-spores swim towards 

 the most highly-illuminated parts, and there in the 

 greatest numbers come to rest, and settle upon the 

 glass, so that, after some four or six hours, on pouring 

 out the water and removing the negative, a print in green 

 swarm-spores can be obtained. The print may be dried,, 

 fixed with albumen, stained, and varnished. The author 

 then dwelt upon the well-known fact that the whole of the 

 animal life upon the globe depends directly or indirectly 

 upon the wonderful synthetic formation of proteid and 

 protoplasm which takes place in the living tissue of 

 plants containing chlorophyll, i.e. green plants, or, to be 

 more exact, in the green chlorophyll corpuscles. He 

 stated that, whatever is the exact chemical nature of the 

 process, this is at least clear, that the first visible product 

 of the assimilatory activity is starch, which, moreover, is 

 found in the chlorophyll grains. The presence of this 

 starch can be made manifest by treating a decolorized 

 leaf with a water solution of iodine dissolved in potassic 

 iodide. This formation of starch only takes place under 

 the influence of light ; the radiant energy of the sun pro- 

 viding the means of executing the profound synthetic 

 chemical change, and building up proteid from the car- 

 bonic acid of the air which is taken up by the leaves and 

 the salts and water of the soil absorbed by the roots. If 

 a plant (and preferably a plant with thin leaves) be placed 

 in the dark over-night, and then brought out into the 

 light next morning, the desired leaves being covered with 

 a sharp and well-developed negative, starch is formed 



' Comptes rendus, cii., 1886, pp. 675 and 1454, ciii. p. 1122. 



2 Ibid., cii p 819. 



3 The reader will find the principal part of Osmond's work in the following 

 papers: Osmond et Werth, "The rie Cellulaire des Proprieies de I'Acier," 

 Ann. des Mines, vii'., 1885. p. 5 ; ," Transformations du Feret du Carbone," 

 Paris, Baudoin et Cie., 1888; "Etudes M^tallurgiques," Ann. des Mines, 

 Juillet-Aout, 1888. There is also a very interesting paper, " Sur le& 

 Nouveaux Precedes de Trempe," which he communicated to the Mining and 

 Metallurgical Congress, Paris, 1889. 



