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filtration, solution, fusion, or distillation are much more rapid processes than on a macroscopic scale. The sensitivity of microscopic tests, expressed in absolute amounts, seems amazing and mystifying. I n relative terms the reactions as carried out are actually less sensitive than if the operations were done in the ordinary way, and the concentrations employed are greater; but since a few utterly unweighable crystals of characteristic form constitute a conclusive test.
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sort in a definit’e amount of sample is counted under the microscope. Shells in cocoa, seed hairs in flour, and bacteria in milk are determined as indices of quality by this procedure. I n mixtures of two constituents, one of which is present in small amount, the percentage composition can be ascertained with fair accuracy by essentially the same method, using a curve prepared from counts on samples having known composition. (Figure 4) Various schemes are employed to insure that like quantities of material are counted in each determination; these range from weighing on a s e n s i t i v e balance to pipetting a suspension of known concentration, or even the admixture of a u n i f o r m l y powdered reference substance, in definite proportion, which indicates the quantity of sample covered by the counting. A more universal type of method depends on counting all the constituents, the particles being “weighted” as to size if necessary, and the composition computed in terms of weight per cent by taking int’o account the specific gravities of the ingredients. (Figure 5 ) I n the case of mixtures of paper fibers factors have been worked out to accomplish this Figure 5-Mixture of Quartz, Orthoclase, Figure 6-Copper C o n t a i n i n g Oxygen result. a n d Plagioclase Feldspars The proportion of total area occupied by dark JVhere the material to be analyzed exists The particles of each mineral are counted and Cu-CuzO eutectic (0.39 per cent oxygen) is a the weight per cent computed. measure of the composition. as a coherent solid, or if particulate can be embedded so as to give a solid aggrethe limiting quantities of reactants are exceedingly minute. gate, a random section through it yields quantitative inUnder ordinary circumstances the analyst is not limited to formation, either by estimation of the proportions of the a n excessively small sample, but in special cases the ad- different solid constituents thus exposed or by precise measvantages of being able to deal with a mere speck or a fern urement of their respective areas. A real analysis is based microscopical fragments, and to secure proofs having forensic on the fact that the intercepts on a surface through a heteropositiveness by putting the sample through a succession of geneous solid are a measure of its volume composition; reactions without exhausting it, are immeasurable. it is extensively utilized in analyses of rocks and alloys. Quantitative microscopical analyses have been developed (Figure 6) in a wide variety of fields, largely because of the need for Conclusion determinations on mixtures of complex constituents and because ordinary chemical methods were too slow or too Each of these glimpses through the microscope might be inconclusive. Even a most elaborate collection of highly accurate numerical results of purely chemical analyses on expanded to a good-sized paper, for each is typical of a nummixtures of papers, textile fibers, food materials, or minerals ber of problems and methods. The analyst’s imagination could be computed in terms of the amounts of the actual and ingenuity, supported b y the available experience of ingredients present only by making a number of hairsplitting previous workers, must be his guide in microscopical studies. Only rarely will he be forced, by limitations of sample, to assumptions. Yet if these ingredients are visibly different res0r.t to them. But continually he will be invited by the under the microscope, or can be rendered so by appropriate rapidity, and above all by the directness, of observations treatment, analyses can be made which, if admittedly under the microscope. And almost never will he be dislimited in number of decimal places, are still adequate as appointed if he resorts to them for, even if his primary regards significant figures. is unanswerable, some additional information is question Here again the directness of the methods and the fact always revealed and some light is thrown on his interpretathat one sees any segregations or non-uniformities in the tions of chemical phenomena. Chemical microscopy offers specimen lend an advantage which is lacking in non-microthe most natural means of extending our visual powers; scopical analyses. The minuteness of the sample required is sometimes of great usefulness, but the rapidity of the by its use we obtain a real “close up” of materials and procoperations, which may often be carried out while the material esses which we ordinarily see only as distant spectators. is in process, is more important. The simplest analyses are based on estimation by direct inspection. Here experience is essential, though the need U. S. Medals for Service in Science for it may b e reduced by having a series of standards for Legislation which is expected to pass Congress a t the next reference. A comparison microscope is useful for such work, session empowers the President to decorate men and women who, since it permits the known and the unknown t o be viewed while in the federal service, have made outstanding contributions simultaneously, side by side in a single field. Where the to science. Two medals are authorized. The Thomas Jefferrequirements are more exacting or where a “scientific guess” son medal of honor for distinghished work in science will be is too prone to unavoidable errors, analyses based on nu- awarded to the scientist who has made a specific contribution t o knowledge of the world. merical determinations are employed. If the material in theThe Jesse W. Lazear medal of honor for distinguished selfquestion-a food, for example-is to be examined for quality, sacrifice will be given to those who risk life and health t o adthe normal number of characteristic particles of a given vance sclence. The medalists would also receive $1000 each. b
