652 



SCIENCE 



[N. S. Vol. XXX. No. 775 



H. ScHOUTEDEN has published another fas- 

 cicle of his work on the Hemiptera of the 

 Congo.' A list is given of all the species now 

 known from that region, nearly 300 in all. 

 The plates illustrate the new species. 



Nathan Banks 



SPECIAL ARTICLES 



THE POSSIBLE EFFECT OF CEMENT DUST ON 

 PLANTS 



A SHORT time ago my attention was called 

 to an extraordinarily abundant deposit of light 

 gray dust on all sorts of exposed surfaces out 

 of doors in one of the valleys not far from 

 San Francisco. This dust was declared to 

 come from the manufactory of Portland ce- 

 ment owned and operated by the Cowell Lime 

 & Cement Co., near Concord, California. 

 Unfortunately the manufacturing plant was 

 shut down during the time in which both of 

 my visits fell, so I did not myself see that the 

 dust came from those works and only from 

 there. I have, however, no reason to doubt its 

 source, the attorney, manager and other offi- 

 cials of the Cowell Company admitting that 

 they lose much cement as dust. 



The light gray dust forms, where reason- 

 ably undisturbed, a film of increasing thick- 

 ness over everything out of doors. This film 

 adheres to some surfaces much more closely 

 than to others, according to the smoothness, 

 hairiness, stickiness, moistness of the surface. 

 It could not be entirely blown, off any surface 

 which I saw, but the rain which fell in the 

 interval between my two visits washed it off 

 some surfaces, but not by any means all. 

 Where the dust fell on undisturbed soil it 

 could be readily recognized because of its color 

 and shade: it is a light gray, whereas the soil 

 is brown. Although the roads are lighter in 

 color than the fields, since they are partly 

 macadamized with a light gray stone, they too 

 are darker than the dust. The origin of the 

 dust is, therefore, clearly not entirely from the 

 roads or fields. 



In composition the dust presents some inter- 

 esting characters under the microscope. It is 



' Ann. Mnsee du Congo Beige, Zool., Ser. III., 

 Sec. II., Tome I., fasc. 1, pp. 88, 2 col. plates, 

 1909. 



evidently composed of fine particles of at least 

 three different sorts. One of these is trans- 

 lucent crystalline fragments, fairly numerous. 

 Another sort, less numerous, consists of some- 

 what larger opaque and fairly rounded par- 

 ticles. By far the largest number, however, 

 are minute granular particles which cohere in 

 irregular masses, often of considerable size. 

 The masses of coherent granular particles en- 

 close and in a way bind together the particles 

 of other sorts. The granular material readily 

 dissolves, with effervescence, in hydrochloric 

 acid, even dilute, but the other particles re- 

 main on the slide, under the microscope, un- 

 dissolved. Acetic acid similarly affects the 

 dust when applied to small quantities on the 

 slide under the microscope. From this it is 

 evident that the dust consists largely of some 

 readily decomposed carbonate. 



This dust more or less completely covers the 

 foliage of the native and cultivated plants in 

 a considerable area, extending, as I observed, 

 to a distance of over six miles from the cement 

 works. It is carried on the winds and, as is 

 so common in this part of California, the 

 winds prevail in very definite directions ac- 

 cording to the season of the year. In conse- 

 quence, the dust goes in one direction mainly 

 during the summer, and leaves the remainder 

 of the valley free. It is more abundant on 

 the windward than the leeward side of scat- 

 tered trees, of orchards, etc. It covers the 

 upper surface of many leaves, such as oak, 

 willow, grape, prune, plum, quince; but such 

 glossy leaves as peach, lemon, orange do not 

 hold it against a breeze. It adheres also to 

 the under side of many leaves, especially if the 

 under side is less smooth than the upper. On 

 fruits it is also evident, especially on dark or 

 dark-skinned sorts, and it can not be removed 

 from them without also rubbing off the bloom; 

 it will not simply drop off if they are dipped 

 or washed in water. 



The market value of property has naturally 

 been influenced by this excess of dust; the 

 salability of land within the affected district 

 being greatly decreased, and the market for 

 otherwise fine table grapes covered by a de- 

 posit of grit is altogether a limited one. 



On the other hand, it may be questioned 



