284 



NA TURE 



[July 21, 1892 



iron. " Graphite carbon exerts an influence only on iron 

 in so far as it diminishes the continuity of the iron molecules. 

 We often meet with the incorrect statement that the influence 

 of carbon on pig-iron is quite different from its action on steel 

 and malleable iron. 



" It is easy to prove to the contrary if we distinguish properly 

 in pig-iron between the combined carbon and that which is 

 only mechanically incorporated as graphite, which ought not to 

 be included in the calculation if we wish to form a judgment 

 on the properties of pig-iron as dependent on its contents of 

 carbon." 



As one understands this, the same applies to steel. 



So far there can be no difficulty in assuming at least the 

 probability of the solution of carbon in iron, and that the 

 physical qualities of the metal are determined by the quantity 

 of carbon in solution, i.e. Akerman's hardening carbon. 



The facts, per contra, appear mainly to indicate that carbon is 

 merely sparingly soluble in iron at temperatures below its fusion- 

 point. 



A more serious objection (previously referred to) is that 

 carbon is practically infusible, more especially in the graphitic 

 form. How this intractable body so readily interpenetrates iron 

 is a problem not easily solved. 



The ordinary chemical theory of solution as usually under- 

 stood does not, however, seem applicable on the whole ; but 

 some of the results accruing from the recent development of the 

 gaseous, or rather physical theory of solution, may be made 

 available for this purpose. 



The Physical or Gaseous Theory of Solution. 



In cases of simple solution the dissolved substance may be 

 regarded as being evenly distributed throughout the solvent. 

 The substance is dissolved by virtue of osmotic pressure, and 

 Van 't Hoff has shown that osmotic pressure in solutions corre- 

 sponds to gaseous pressure in space. 



Further, it appears that both Boyle's and Charles's law holds 

 good, at least for dilute solutions, osmotic being the equivalent 

 for gaseous pressure, which pressure increases for constant 

 volume proportionally to the absolute temperature. It has 

 been, however, objected that Boyle's law is not strictly applic- 

 able to " more especially concentrated solutions," but Prof. 

 Orme Masson (Nature, February 1891), states that these are 

 comparable with the case of gases at high pressures. Again, ex- 

 ceptions are claimed under the law of Avogadro, i.e. equal 

 volumes of gases contain equal numbers of molecules under 

 like conditions of temperature and pressure, but as regards 

 compound gases exceptions occur, as also with dilute solutions. 



Exceptions can be explained by the theory of dissociation. 

 The analogy between gases and the physical theory of solutions 

 thus seems complete, and Ostwald describes an experiment in- 

 dicating the existence of free ions in a dilute solution of 

 potassium chloride ; other instances might also be quoted. 



The author's object, however, is not to discuss the absolute 

 correctness or otherwise of the theory of gaseous solution, which 

 seems pretty well established ; but to show that it may be ap- 

 plicable to the solution of carbon in molten, semi-fluid, or even 

 merely heated iron, apart from possible cases of dissociation and 

 chemical combinations. Solution is simply the even distribution 

 of one body in another, or such distribution as that of per- 

 manent gaseous matter through space. It may be urged that 

 the theory is not applicable to semi-fluid or merely heated iron. 

 No definite line can, however, be drawn ; it is obvious that the 

 <iifferent grades of temperature are simply approximations, more 

 or less, to the ideal fluid condition. The law of solution, 

 as above defined, may suffer modifications, but need not in con- 

 sequence be rejected. 



' ' Defiiiition of Osmotic Pressure. ^ 

 " Osmotic pressure is really a definite force. With suitable 

 apparatus this force can be measured, in centimetres of a 

 mercury column, and Pfeffer- has shown that this, the osmotic 

 pressure, is intimately connected with the nature of the dissolved 

 substance. 



" The pressure was found to be dependent on, and in propor- 

 tion to, the concentration of the solution ; the pressure at a 

 specified concentration is dependent on the temperature — a rise 

 in temperature corresponds to an inci-ease in preisure. 



" This discovery remained unnoticed. In the first instance the 



I Ostwald, " On Solution." 

 NO. 1 186, VOL. 46] 



facts were only required for the elucidation of certain physio- 

 logical questions. 



"And it was not until 1886 that Van 't Hoff developed a 

 theory of solution based on these phenomena. 



" Osmotic pressure is a specific property of the substance in 

 solution, and in this respect resembles gaseous pressure. The 

 analogy between the state of solution and the gaseous state is 

 clearly shown (pp. 1 15-17). Dissolved substances exert the 

 same pressure in the form of osmotic pressure as they would 

 exert if they were gasified at the same temperature without 

 change of volume. 



*' All that we know of gases holds good for solutions, substi- 

 tuting osmotic for gaseous pressure. 



" Osmotic pressure is, in some instances, very great." 



And it seems clear that osmotic pressure is not a mythical, but 

 a real or actual force of considerable power, and one which may 

 be rationally applied to the elucidation of the cause of the car- 

 bonization of iron ; further, it may even aff'ord a clue to the 

 phenomena observed in the production of other alloys. 



As regards the carburization of iron, the physical theory of 

 solution, " founded on the identity of osmotic with gaseous 

 pressure," seems the only one capable of affording a satisfactory 

 explanation of the facility with which carbon combines with iron. 



The chemical, or old, theory of solution apparently fails to do 

 this. The same may be said of the assumption that chemical 

 combinations of iron and carbon are formed. Although it must 

 be granted such combinations may exist, yet, in the author's 

 opinion, complete proof is still wanting. It is really difficult to 

 realize, when dealing with stable bodies like iron and carbon, 

 how their union can be thus accomplished. 



On the contrary, the application of the law of osmose renders 

 the conception of the transfer of carbon to iron very easy. 

 This force, exerting probably almost illimitable power in nature, 

 seems the only one capable of overcoming the inertia of 

 bodies ; such, for instance, as that of iron and carbon. 



The physical theory of solution has hitherto only presumptively 

 herein been applied to the solution of solids in liquids ; and 

 it may be asked, Is it applicable to the case of the solution of 

 solids in solids, such as carbon and ii'on, when heated ? 



To this one can reply with confidence that the absolute solid 

 has no existence. Unless we reject the atomic theory, it is 

 evident that no tangible mass of matter can be termed a solid : 

 it is an agglomeration of atoms. Further, accepting the defini- 

 tion of what is termed the atomic volume — i.e. the spaceoccupied 

 or kept free from the access of other matter, by the material 

 atom itself, together with its investing sphere of heat — it follows 

 that the atoms must be apart from each other in the so-called 

 solid mass, and the distances between the atoms are probably 

 considerable as compared with the actual volume or size of the 

 atoms themselves. Therefore, there can be no difficulty in con- 

 ceiving that osmotic pressure plays a part in the case of a mass 

 of matter, "conventionally termed a solid." It is only a ques- 

 tion of degree ; the quantity of matter dissolved in a given 

 time is simply a function of the temperature applied, and at 

 a low temperature, the effective osmotic pressure in the case 

 of solids seems comparable to that of a liquid evaporating under 

 pressure of its own vapour. Evaporation is retarded, and 

 the analogy may hold good in the case of the conventional 

 solid. John Parry. 



PHOTOMETRIC OBSERVATIONS OF THE SUN 

 AND SKY.-^ 



A TTEMPTS have been made by Clausius and various other 

 "^ mathematicians to calculate the light at different points of 

 the perfectly clear sky, and to compare the light of the whole (or 

 a portion) of the sky with that of the sun. The difficulties of 

 photometric measurement have prevented any of the theories 

 being thoroughly established by experimental verifications. 



In the first period of photography, it became a matter of 

 practical importance to have some way of testing roughly the 

 "actinic activity of diffused daylight," in order to obtain a 

 guide for the time of exposure. Very many photographers, in 

 those days when the evils of over-exposure could not be corrected 

 in the printing, must have exposed a scrap of sensitive paper, 

 and thence concluded how many seconds' exposure they would 

 allow. 



' " Photomelric Observations of the Sun and Sky," by Wm. Brennand* 

 Proceedings of the Royal Society, vol, 49, n. 2S8, .April 18, 1891, pp. 255- 

 280. 



