Texas Gypsum Formation

Duncan H. Cummins
1892 Science  
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more » ... out Early Journal Content at http://about.jstor.org/participate--jstor/individuals/early-journal--content. JSTOR is a digital library of academic journals, books, and primary source objects. JSTOR helps people discover, use, and build upon a wide range of content through a powerful research and teaching platform, and preserves this content for future generations. JSTOR is part of ITHAKA, a not--for--profit organization that also includes Ithaka S+R and Portico. For more information about JSTOR, please contact support@jstor.org. SCIENCE. SCIENCE. salt, are like, and that is perhaps the reason that so little attention has been paid to the definition 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 characterized 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 suspensions or emulsions 1; 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 extremely small emulsified or suspended particles and the liquid may be so great that, in comparison with it, gravity vanishes. According 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 homogeneous 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 temperature 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.