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POPULAR SCIENCE MONTHLY. 



tor has been steadily at work endeavoring to 

 perfect his method, and it is now announced 

 that he has succeeded in eliminating those 

 irritating substances which were so trouble- 

 some in the early preparations. From his 

 observations Koch concluded that the affected 

 patient gradually acquired a degree of im- 

 munity through the absorption of certain 

 constituents of the bacilli themselves. This 

 immunizing, however, usually occurs too late 

 in the disease to save the patient. In or- 

 der to collect these substances, whatever 

 they may be, he dries and triturates the dead 

 bodies of the bacilli, and from the resultant 

 powder makes two extracts. The first of 

 these is opalescent, and is practically iden- 

 tical with his original serum. He calls it 

 tuberculin (TO). The remaining sedi- 

 ment is again dried, digested with water, and 

 then centrifuged, and this is continued until 

 the water is perfectly clear. These later 

 extracts contain the essential immunizing 

 principle which he calls tuberculin R (TR), 

 and with which such encouraging results are 

 said to have been obtained. 



Some Notes on a Dnst Storm. Apropos 

 of our recent article on Dust Storms, we have 

 received the following item from Mr. W. S. 

 Jackman: "On the 19th of February, 1896 

 (Ash Wednesday, by eternal fitness), a remark- 

 able dust storm descended upon the city of 

 Chicago. As the ground had been complete- 

 ly covered with snow to the depth of several 

 inches a day or two previous, it was an easy 

 matter to form some estimate of the quan- 

 tity of dust that fell. The storm began in 

 the evening and lasted several hours. The 

 next morning, by selecting suitable open 

 areas that were likely to be free from eddies 

 and local currents, an average per square 

 yard of 1*34 ounces of dust was obtained. 

 This was gathered by scraping up the snow 

 to the depth the dust had penetrated about 

 one inch and, after melting it, the water 

 was evaporated and the dust dried. The 

 amount thus collected measured 2*45 cubic 

 inches. At this rate the quantity of dust 

 deposited upon a square mile would be about 

 129*6 tons; the volume would be on the 

 same area 4,352 cubic feet, equal to a pile 

 about thirty-four feet long, sixteen feet wide, 

 and eight feet deep. On being strongly heat- 

 ed in a clay crucible the dust turned reddish 



brown and lost twelve per cent of its weight. 

 The microscope showed it to be largely com- 

 posed of irregular and rounded quartz grains. 

 As the storm began after dark, people who 

 were unfortunate enough to be caught in it 

 were at a loss for a time to know just what 

 was the matter. In many instances nothing 

 unusual was suspected until, entering their 

 homes, their smeared and blackened counte- 

 nances called forth an unwontedly hilarious 

 greeting. After the snow melted, in many in- 

 stances the sidewalks were so covered with 

 the slimy mud as to need scraping aijd 

 washing. The storm seemed to come from 

 the northwest, and was accompanied by a 

 moderate gale." 



The Law of Mississippi Floods. An im- 

 portant point in Mr. James L. Greenleafs 

 study hi the Engineering Magazine of the 

 Times and Causes of Western Floods is 

 the topographical division of the country 

 drained by the Mississippi into several large 

 water sheds, covering a total area of 

 1,259,000 square miles. Although this topo- 

 graphical division has been wrought by 

 natural causes, the consideration of the 

 water sheds must be combined with that of 

 climatic areas for the purposes of the pres- 

 ent study. All the rivers tributary to the 

 Mississippi show a decided tendency to low 

 water in the autumn, the southern waters 

 beginning to fall from a high stage in June 

 and the northern in July. All begin to rise 

 from low water in winter the southern 

 tributaries earlier, and the northern ones 

 later. Two freshets occur during the year 

 in each of the large tributary basins. The 

 coincidence of the highest stage in more 

 than two of the large branches is extremely 

 rare, and hence it is an exception for the 

 main river to be subjected to the enormously 

 congested state which would otherwise result. 

 The varying stages of the Mississippi accu- 

 rately reflect the fluctuations of rainfall and 

 of temperature which occur upon its tribu- 

 tary basins, and these are followed in detail 

 in the author's paper. The Mississippi being 

 virtually created by the union near one point 

 of three large rivers the Ohio, the Mis- 

 souri, and the upper Mississippi these three 

 branches, of which the Ohio is regarded as 

 the most important, naturally stamp their 

 characteristics upon it to a very marked de- 



