HAEDWICKE'S SCIENCE-GOSSIP. 



NOTES ON PLEOMORPHISM. 



By P. Duffy, E.L.S., E.C.S. 



1~N philosophical language the term identity indi- 

 -*- cates the sameness of a substance or thing 

 under every possible variety of circumstances. This 

 is the abstract definition of identity, but in nature 

 we can no more find it realized than we can find 

 realized the mathematical definition of a straight 

 line. On the contrary, 1 am inclined to think that 

 the more extensive our knowledge may be of the 

 facts which nature reveals to our observation, the 

 more embarrassed we become in forming the con- 

 ception which the abstract . definition of identity 

 suggests, for experience teaches nothing more 

 impressively than that the slightest change in the 

 environment of any material body is accompanied 

 by a corresponding alteration in its properties. 

 Thus, whenever we alter the conditions of light, 

 heat, electricity, and gravitation, by which any 

 substance is surrounded, do we not produce some 

 correlative change in the qualities of that sub- 

 stance? And is it not equally true that under any 

 given set of circumstances almost every substance 

 will assume the same condition that belongs to it 

 at any other time under that same set of circum- 

 stances ? If we heat a piece of platinum, for in- 

 stance, and after heating allow it to cool to the 

 temperature with which we started, we find that it 

 resumes all its original qualities, and that, provided 

 no chemical reaction be allowed to intervene, its 

 bulk and its specific gravity, which is the same 

 thing in other terms, and „its relations to heat, 

 light, and electricity will be just the same as those 

 it possessed before we subjected it to heat. Every 

 test of identity that we can now apply to it elicits 

 but one uniform answer, viz., that it is the same 

 substance; and this result is perfectly in accord- 

 ance with what our ordinary conception of identity 

 in nature leads us to expect. But we should err 

 greatly if we were to conclude that every material 

 substance conforms rigidly to these notions. So 

 far is this from being the case, that there are many 

 bodies, several solid and some liquid, which admit 

 of a variation of properties, and present different 

 appearances under the same conditions of tempera- 

 ture, light, and so forth. Most persons are ac- 

 quainted with the two forms of carbonate of lime in 

 calc spar and arragonite, which not only belong to 

 different crystalline systems, but differ in density, 

 and are further distinguished by the peculiar 

 property possessed by^calc spar of doubly refracting 

 a ray of light. Now, although there is, as we see, as 

 great a difference between calc spar and arragonite 

 as there is between many bodies that are in no 

 sense identical, it is a fact that we may transmute 

 these two substances one into the other, and 

 chemistry can point out no difference in the ponder- 

 able elements of which they consist. Thus, if we 



heat a crystal of arragonite in the flame of a spirit- 

 lamp, it falls into a powder composed of calc spar: 

 while if we add a solution of Ca.Cl. to NH 4 C0 3 , 

 we obtain a granular precipitate, the particles of 

 which have the form of calc spar or of arragonite, 

 according as the temperature of the solution is 50° 

 or 150° E. ; and whether the crystal be one of calc 

 spar or one of arragonite, all that chemical analysis 

 tells us about it is, that every 100 parts of it con- 

 sist of 40 of calcium, 12 of carbon, and 48 of 

 oxygen. A substance which has thus the power of 

 assuming different and incompatible crystalline 

 forms is [said to be dimorphous, trimorphous, or 

 polymorphous, according as the number of forms is 

 two, three, or many. Although these terms, de- 

 rived from the Greek word nop$i\, form, and a prefix 

 expressing the proper numeral, are not generally 

 understood in chemistry to denote any other than 

 the number of crystalline forms which a solid body 

 is capable of assuming, I think their meaning may 

 without any impropriety be extended so that they 

 shall be understood to refer to every condition of 

 the same substance in the solid state, whether 

 crystalline or destitute of crystalline character. 

 In biological science the term pleomorphism is 

 employed to denote the actual or possible occur- 

 rence of any number of forms more than one, of an 

 individual animal or plant, or of a species of animals 

 or plants. Eor convenience in grouping together 

 phenomena that have at least this feature in com- 

 mon, I intend, in the remarks I have to make, to 

 employ the term pleomorphism in this sense, 

 when speaking of ^non-living as well as of living 

 matter. 



We have no difficulty in meeting with illustra- 

 tions of the phenomenon I refer to, either in the 

 inorganic or organic kingdom of nature. I may 

 begin with oue that is familiar to us all. I do not 

 assume that any of us first made his examination of 

 barley-sugar in a spirit of science, but, whether 

 or not, he can hardly fail to have observed that the 

 sticks of this substance, which are at first clear and 

 glassy, become after a time white and opaque ; they 

 pass, in fact, from the glassy condition into the 

 granular or crystalline one, the change occurring 

 without the slightest loss or gain of weight. This 

 change may also be reversed, the vitreous condition 

 of the substance being restored by merely melting 

 it by heat and allowing it to solidify. Now the 

 difference in the physical properties of the sugar in 

 the one condition and in the other is as great as can 

 be discovered between many bodies that do not, 

 approach each other in composition; yet any one 

 who investigates the subject will find that one and 

 the same substance underlies the two different 

 conditions. 



The transition from the glassy to the granular 

 condition may be effected quickly if, instead of keep- 

 ing the sugar at the ordinary temperature of the air, 



