182 



THE AMERICAN MONTHLY 



[October, 



By a somewliat rough measurement 

 of the temperatures obtained with 

 this apparatus, I find it possible to 

 procure a range of about 'i5°C. 

 81°F.), or from 27oC. (BO.eoF.) to 

 Y2°C. (I6I0F.) 



As the normal temperature of 

 the human bodj is 37°C., it will be 

 seen that the range is all that is 

 needed for the object for which it 

 was intended. The higlier tem- 

 peratures are convenient for favor- 

 ing chemical reactions under the 

 microscope, or for the evaporation 

 of liquids, or other uses where a 

 gentle heat and uniform tempera- 

 ture are desired. 



The Movement of Diatoms. 



All persons who have observed 

 living diatoms are familiar with 

 their curious forward and backward 

 motion, but although many have 

 endeavored to determine the cause 

 of these movements, the subject is 

 still very imperfectly understood. 

 Mr. Julien Deby's excellent article 

 entitled, " What is a Diatom " has 

 been widely known and read by 

 microscopists, both here and in Eu- 

 rope, but the letter which Prof. H. 

 L. Smith wrote to Mr. Deby, soon 

 after his article was published, is 

 not so well known.* 



By the use of a pigment in the 

 water, Prof. Smith has been able to 

 study the movement of diatoms to 

 great advantage. The color em- 

 ployed was the ordinary indigo-blue 

 used by water-color painters. 



If a living Pinnvlaria be placed 

 in water colored by that pigment, 

 and examined with the median line 

 turned toward the eye, the little 

 particles of indigo will be seen 

 to move along the line, and to 

 accumulate near the centre in a mi- 



* Prof. Smith's letter is published in the 

 " Annals" of the Belgium Microscopical So- 

 ciety, Vol. IV. 



nute ball. Viewed with the con- 

 nective turned toward the eye, it 

 will be observed that a ball of in- 

 digo accumulates about the centre 

 of each valve, and both balls turn 

 about on their axes, just as they 

 would if a minute jet of water were 

 directed against each from an orifice 

 situated at the central extremity of 

 the median line of each valve. When 

 the balls have attained a certain 

 size they suddenly burst, and the 

 particles of indigo pass on along 

 the median line in a direction op- 

 posite to that in which the diatom 

 is moving. Immediately after the 

 rupture of the ball, a new one be- 

 gins to form. The movement of 

 the particles along the median line 

 always takes place in a direction 

 opposite to the motion of the diatom. 

 It is to be hoped that observations 

 of this kind will be continued by 

 others. There are some very cu- 

 rious appearances to be seen in the 

 examination of moving diatoms, 

 and as any person who has patience 

 and good objectives, can readily 

 study the subject, we may hope for 

 an early and a satisfactory explana- 

 tion of the cause of the motion. 



Apple and Pear Blight.* 



The wide-spread and disastrous 

 disease of the pear tree, commonly 

 known as fire-blight, and that known 

 as twig-blight of the apple tree, are 

 due to the same agency. They are 

 identical in origin, and as similar in 

 pathological characteristics as are 

 the trees themselves. The quince, 

 Lombardy poplar, American aspen, 

 and some other trees suffer from the 

 same or similar affections. The im- 

 mediate cause of the disease is a 

 living organism, which produces 

 butyric fermentation of the material 

 stored in the cells, especially those 



* Abstract of an article by Prof. T. J. 

 Burrill, read at the meeting of the A. A. A. S. 



