December 23, 1892.] 



SCIENCE. 



353 



salt, are like, and that is perhaps the reason that so little atten- 

 tion has been paid to the deBnition of solutions; what every one 

 has a clear idea of, hardly needs defining. But when we come 

 to speak of solutions of colloids, difficulties arise. It is not hard 

 to distinguish true solutions of crystalloids, for they are charac- 

 terized by the circumstance that for every temperature there is 

 a fixed and constant ratio between the quantities of substance 

 dissolved and solvent. But when we come to apply this criterion 

 of solubility to colloid solutions, we find it insufficient. Some 

 maintain that such solutions are in reality nothing but suspen- 

 sions or emulsions '; and indeed this may be true in certain cases, 

 for there exist as wide differences between colloids and colloids 

 as between crystalloids and colloids. But the question at issue 

 is, Can a suspension or emulsion remain perfectly homogenous 

 for an indefinitely long time? The question can be answered in 

 the affirmative in the case of suspensions or emulsions in which 

 the suspended or emulsified particles have the same density as 

 the suspending or emulsifying liquid. This is an extreme case, 

 it is true. Still it proves that there may be entirely homogeneous 

 mixtures which are certainly not solutions. 



Again, it may be said that the surface tension between the ex- 

 tremely small emulsified or suspended particles and the liquid 

 may be so great that, in comparison with it, gravity vanishes. 

 Accoi-ding to this, even if there existed a difference of density 

 between the particles and the liquid the emulsion or suspension 

 would remain as such indefinitely. Their exist then homo- 

 geneous mixtures that may not be true solutions. 



Further, under certain conditions, a true solution may become 

 heterogeneous. If one part of a solution be at a different tem- 

 perature or pressure from another, diffusion will take place and 

 the solution will cease to be homogeneous. 



With reference to the third question, probably all will agree in 

 understanding by mechanical means, in this connection, filtration, 

 subsidation, etc. In regard to subsidation, it has been shown 

 above, that many emulsions and suspensions do not subside even 

 after the lapse of a long time, so that this criterion fails in this 

 respect. But let us see if we cannot separate a solution into its 

 constituents by means of filtration. Take a solution of casein in 

 dilute sodium carbonate, for instance. This passes quite freely 

 through ordinary filter-paper; but if the paper be converted into 

 parchment paper, although the sodium carbonate still passes quite 

 freely through its pores, the casein is retained. If now ai amor- 

 phous precipitate of ferrocyanide of copper be deposited in the 

 parchment paper, even the salt is kept back, only the water being 

 able to pass through the interstices of the precipitate. Thus by 

 mechanical means a solution has been resolved into its component 

 parts. 



We conclude, then, that the existing definitions of solutions are 

 inadequate; it remains to propose another more in accordance 

 with fact. 



Scientific definitions generally consist in the statement of cer- 

 tain attributes that separate as by a boundary the thing to be 

 defined from all other things. If, then, there exists some at- 

 tribute of solutions which is ever present, and indeed character- 

 izes them as such; if other attributes are but different modes of 

 expressing this essential attribute, such an attribute can well 

 serve to define solutions. An attribute that fulfils the above 

 conditions is the osmotic pressure. A solution is accordingly a 

 homogeneous mixture exerting an osmotic pressure. 



It is, of course, assumed that temperature and pressure are 

 constant, else a solution might cease to be homogeneous. As 

 osmotic pressure is a term applied only to mixtures in the liquid 

 or solid state, it follows that "gaseous solutions" do not exist. 

 For a •' solution" of a gas in a gas, mixture is much the better 

 term, and is indeed in common use; for a solution of a liquid in 

 a gas, the proper word is still mixture, as well as for the rare 

 case of the " solution " of a solid in a gas. 



That osmotic pressure is the true criterion of solutions has 

 strict scientific warrant. As soon as the conception of a pressure 

 in solutions analogous to that in gases was gained, a great stride 

 in advance was made. The most striking properties of solutions, 



1 See my paper " Oa the Nature of Colloid Solutions " In American Journal 

 of Science for March, 1892. 



diffasioa, lo'vering of the freezing point, raising of the boiling 

 point, are directly due to osmotic pressure; hence if osmotic 

 pressure be predicated of solutions, it is implicitly stated that 

 they diffuse, boil at a higher and freeze at a lower temperature 

 than the solvent. All other properties of solutions are also more 

 or less directly referable to osmotic pressure. The definition 

 proposed is, therefore, entirely adequate, sharply separating 

 solutions from all other mixtures. 



TEXAS GYPSUM FORMATION. 



BY DUNCAN H. CUMMINS, AUSTIN, TEXAS. 



Prominent among the strata composing the Permian formation 

 in Texas, are the Gypsum Beds, which, taken with those of the 

 north-west, are the most extensive of any such formations in 

 the world. The Texas beds extend over an area of upwards of six 

 million acres. Extending from the north line of the State, south, 

 to the line of the Texas and Pacific Railroad, the beds vary in 

 thickness from that of a sheet of paper up to seventy-five feet. 

 The east line of the deposit passes Sweetwater, on the line of 

 the Texas and Pacific Railroad, in Nolan County. The west 

 line passes about twenty miles east of the Staked Plains. The 

 greatest thickness of these beds is about nineteen hundred 

 feet. 



There are six forms of gypsum to be found in these beds, all 

 contain the same chemical ingredients, but differ in their manner 

 of crystallization: selenite, rose, massive, radiated, and fibrous 

 gypsum, and alabaster. ' 



The selenite is a clear, transparent variety, and may be split 

 into very thin slices. Excellent cabinet specimens of this variety 

 may be found in the red clays near Guthrie, in King County. 



Rose gypsum is a foliated selenite, found only in one place in 

 this belt, so far as has been reported, and that near Sweetwater, 

 in Nolan County. The plates are fixed in the form of a rose and 

 are so called by the people of that vicinity. 



Massive gypsum is the principle form of which these beds are 

 composed, this form occurs in beds of varying thickness at dif- 

 ferent horizons, ranging in thickness from one inch to seventy- 

 five feet throughout this belt. It is generally white in color, but 

 often it possesses a blue or reddish cast. 



The radiated variety is usually round in figure, the lines of 

 crystallization diverging from a common centre. This form 

 possesses high specific gravity. 



Fibrous gypsum, or satin spar as it is sometimes called, occurs 

 in white or slightly colored deposits throughout this belt. Very 

 few of these seams exceed two inches in thickness, although there 

 may be seen in the museum of the Texas Geological Survey 

 blocks of this form, from Kent County, exceeding twelve inches 

 in thickness. 



Alabaster occurs in many localities throughout the gypsum 

 belt. Its beauty as a cabinet specimen is due to its color and 

 translucent structure. It may be carved into many ornaments, 

 and is capable of receiving a high polish. 



Besides these six distinctive forms, the gypseous marls and the 

 heavy beds of gypsiferous sandstones occur in great abundance 

 throughout this area. Many of the above-mentioned forms have 

 beautiful combinations and weatherings. Noticeable among these 

 are a puddingstone gypsum, a combination of blue and white 

 massive gypsum, a striated form composed of alternating layers 

 of red and white massive gypsum, and a form of alabaster ex- 

 hibiting very peculiar weatherings, grooves being washed in many 

 directions on its surface, also a beautiful cabinet specimen from 

 King County, it b'3ing round nodules of alabaster or selenite with 

 a heavy incrustation of carbonate of copper. 



Ou account of the scarcity of transportation, no uses are being 

 made of these vast beds, which are unexcelled for use as fertiliz- 

 ers, or the manufacture of plaster of Paris. 



In conclusion, to the scientist, Texas presents opportunities for 

 study excelled by no place. Her geological and mineralogical 

 products are subjects for discussion the world over, and no pret- 

 tier field is open for investigatioa than the Texas gypsum forma- 

 tion. 



