762 



THE POPULAR SCIENCE MONTHLY. 



with carbonic acid, and the only means of 

 destroying that poisonous gas is found in 

 plant-agency. Hence, if the atmosphere of 

 a city were to be inclosed within imperme- 

 able walls, and there were no growing plants 

 within the inclosure, the air would quickly 

 become irrespirable. But of course the 

 air is nowhere thus walled about, and hence 

 the deleterious gases it contains are dissi- 

 pated and carried away by the unceasing 

 movement of the atmosphere to other re- 

 gions where an abundant vegetation may 

 deprive it of its carbonic acid. Still, there 

 is no doubt that this purification of the air 

 is accelerated by the presence of vegeta- 

 tion in the cities themselves. The writer 

 in the Garden asserts that " Paris has now 

 so large a number of parks, and its streets 

 and boulevards are so profusely planted 

 with trees, that the death-rate has been 

 thereby reduced from one in thirty-four as 

 it formerly was, to one in thirty-nine as it 

 now is." 



But trees are further of service in shad- 

 ing gutters and road-ways, thus materially 

 retarding and preventing the action of the 

 sun in producing noxious fermentation. 

 Then, too, the roots of the trees take up large 

 quantities of such matters as are washed by 

 the rains .into the interstices of the pave- 

 ments. Besides these direct sanitary bene- 

 fits, we must also take note of the comfort 

 derived from the shade of the sidewalks. 

 Last, though not least, the beauty of our 

 cities would be greatly enhanced by the 

 planting of trees in the streets. The author 

 recommends the planting of the sunflower 

 on the Harlein flats of this city. By this 

 means the poisonous gases arising from the 

 decaying garbage used for filling these flats 

 would be neutralized far more effectually 

 than by the application of either " injunc- 

 tions or disinfectants." 



Magnetism and the Imagination. Dr. 



Volpicelli, in a communication to the French 

 Academy of Sciences, describes certain ex- 

 periments made by him to determine whether 

 a magnet can have any influence upon per- 

 sons of nervous constitution. The first per- 

 son experimented on was a patient of the 

 hospital Santo Espirito, in Rome, whom the 

 sight of a magnet was sufficient to throw 

 into convulsions. Volpicelli brought with 



him a simple piece of unmagnetized iron ; 

 this, however, produced all the effects at- 

 tributed to the magnet. The second experi- 

 ment was made on a person similarly af- 

 fected with nervous disorder. Volpicelli 

 placed a magnet in this person's hand, and 

 soon the super-excitation was such that it 

 had to be taken away. A few days later 

 the subject of this experiment presided at 

 a meeting of scientific men. All unknown 

 to him, magnets had been introduced into 

 his chair, into the drawer of his table, under 

 his feet in short, all around him. The 

 meeting lasted for two hours, and, at its 

 close, on being asked how he felt, he de- 

 clared that he was perfectly well. " It ap- 

 pears to me," continues Dr. Volpicelli, 

 " that these two experiments are sufficient 

 to prove that magnetism has no effect upon 

 the nervous system, and that the cause of 

 the effects produced by the presence of a 

 magnet is to be attributed only to the ima- 

 gination. As I have shown, if we bring one 

 or more powerful magnets near to a patient 

 without his suspecting their presence, no 

 appreciable effect is produced. For the 

 physiologist, the most interesting circum- 

 stance connected with these experiments is 

 the diversity of effects produced by the ima- 

 gination in nervous subjects when they see 

 a magnet, or suppose the presence of one. 

 The diversity of these effects will, perhaps, 

 lead to the discovery of some new truths." 



A Lost Species rediscovered. How sad 



the idea of the loss of a species ! Suppose 

 our robins were reduced to a single living 

 specimen ? When inevitable death should 

 come, the going out of that one individual 

 life would be the extinction of its race for- 

 ever. There is the typical fact of the dis- 

 appearance of the dodo. And at home we 

 have the equally remarkable fact of the 

 extinction of that noble shore-bird, the 

 great auk. It is now fifty-five years since 

 Major Long's expedition returned from the 

 Rocky Mountains, bringing many unknown 

 forms of life. Of this expedition Thomas 

 Say was chief zoologist. Among the many 

 new species was one especially of the Ci- 

 cindda, or tiger-beetles, those beautiful in- 

 sects which have always been favorites with 

 the entomologists. Say described, and 

 named it Cicindda limbafa. At that time 



