164 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



form, so the expansion by heat may not take place instantly, 

 but continue itself after the change of temperature. A few 

 observations with glass rods, by Matthiesen, seem to point in 

 this direction. Probably this thermal after-action, like the 

 elastic, occurs in an eminent degree in organic substances. 

 1A, XL VII., 157. 



THE TRUE ZERO POINT OF THE CENTIGRADE THERMOMETER. 



Dr. Craig, in a communication to the American Chemist, 

 calls attention to the fact that the true zero point of the 

 Centigrade, or 32 of the Fahrenheit scale, is not that of the 

 freezing-point of water, but the melting-point of ice, and that 

 water in winch even a considerable quantity of ice is im- 

 mersed is usually several degrees above this point. If, how- 

 ever, the bulb and stem be immersed in melting ice or snow, 

 from which the water is draining off as it is produced, the 

 mercury will attain the exact temperature desired. Dr. 

 Craig further remarks that any vessel filled with clamp snow 

 will give correct results, if the instrument is driven clown in 

 the snow up to the 32 point, and a lens is placed on the top 

 of the snow to read by, provided there is no accumulation of 

 water about the bulb. He also refers to the fact that there 

 is a gradual change in the scale of the thermometer with age, 

 the reading becoming generally too high. This is due to the 

 contraction which in time takes place in the bulb, although 

 occasionally there may be temporary changes in the opposite 

 direction. 1 A, July 24, 1874, 33. 



THE EFFECT OF LIGHTNING ON TREES. 



The theory that the splitting of the trunks of trees by 

 lightning is the result of the sudden evaporation of the liq- 

 uids contained within them has received much confirmation 

 from experiments made by Osborn Reynolds, who succeeded 

 in splitting small sticks of wood by passing the electric spark 

 through them after they had been impregnated with water. 

 He also burst small glass tubes which were filled with water, 

 although the same tubes, when empty, allowed the electric 

 spark to jump through them without in the least disturbing 

 them. The most striking experiment made by him was upon 

 a tube three eighths of an inch exterior and one eighth inte- 

 rior diameter, which could stand a pressure of at least two 



