The Microscope. 243 



These four slides were selected from a collection of several 

 hundred, all testifying to the truth of the foregoing remarks, 

 and exhibiting a feature of which we have not spoken and upon 

 which we hazard a theory, viz: That all those trees growing in 

 sandy or dry soil, presented fewer and smaller cell-marks than 

 those found in rich and moist ground. It is a known fact that 

 the rock maple — acer saccharinum — yields sap in quantities de- 

 pending upon the conditions of its growth, and in view of the 

 fact that these cell-markings are the avenues by which the sap 

 rises in the tree, it is our belief that they will vary according 

 to the amount of sap flowing, which in turn depends upon the 

 locality. 



METHODS OF STUDY. 



ALBERT E. JENKINS, ANN ARBOR, MICH. 

 FIXING AND HARDENING. 



PICROSULPHURIC ACID. (KLEINENBEKG'S FLUID.) 



THIS is prepared by Dr. Mayer as follows : 

 Water, distilled, 100 vols. 



Sulphuric acid, 2 vols. 



Picric acid in crystals, as much as will dissolve. 



This is the most extensively used of all the killing fluids ; 

 but it is especially valuable in embryological and invertebrate 

 histological study. It may be used with good results everywhere 

 except with objects possessing calcarious parts, as it dissolves all 

 forms of calcium and precipitates it as crystal of calcium sul- 

 phate or gypsum, which render the object unfit for delicate sec- 

 tions. If it is desirable to use picric acid as a killing fluid on 

 such an object it should be used alone as it thus retains the dis- 

 solved lime in solution ; or picromitric acid, the formula for 

 which appears on another page, may be employed instead. 

 Picrosulphuric acid causes the connective tissue of vertebrates 

 to swell, and this fact should be remembered in its use ; the ad- 

 dition of creasote, which was proposed as a remedy for this un- 

 desirable feature, does not have any appreciable effect. Picric 

 acid and all its combinations imparts to objects treated with it 

 an intense and lasting yellow color which, unless quite removed, 

 interferes with the ease and durability of the staining ; and as it 



