208 SOME REMARKS ON CLARIFICATION, ETC. 



transparent, does not become cloudy through the addition of 

 concentrated glycerine, and remains, when treated with water, 

 moderately translucent. The swelled fecula reacts with iodine in 

 both cases, but producing two different colours, viz. : the chloral 

 hydrateared, and the salicylate a pure blue. Salicylate with iodine 

 would, for this reason, be valuable for discovering traces of starch. 

 Salicylate in general has the advantage of containing a neutral, 

 non-corrosive, non-poisonous, and non-hygroscopic salt, its principal 

 chemical virtue being an extraordinary power for swelling starches. 

 The chief feature, however, which renders salicylate so admirably 

 useful in microscopy, is its miscibility with phenols, notably oil of 

 cloves. If one drop of oil of cloves be mixed with lo of salicylate 

 an opalescent liquid will be obtained, which becomes clear by 

 adding a second drop of oil of cloves. If this process of adding 

 oil of cloves be continued, about the 20th drop will produce a 

 cloudy solution, which may be clarified by a single drop of salicylate. 

 A liquid may thus be obtained with a refractive index of i'5, which 

 is about that of cellulose. 



Weather-Charts. — Atmospheric pressure, and consequently 

 the height of the barometer, varies not only from place to place, 

 but also, in any one place, from day to day, and even from hour to 

 hour. All those movements of the air that we know as winds are 

 due to more or less local differences of atmospheric pressure ; varia- 

 tions in the force and direction of the wind are among the chief 

 factors in weather ; and so the barometer, by showing whether the 

 pressure is on the increase or on the decrease, whether rapidly or 

 slowly, enables us to foretell changes of weather. The actual height 

 of the mercury being of far less importance than the direction and 

 rate at which it is moving, the words "change," "fair," "set fair," 

 etc., usually placed by the side of the instrument, are of little value. 

 The chief points in weather-charts are, therefore, the readings of the 

 barometer at various places at short intervals of time — usually daily. 

 Lines joining places which have at a given time the same baromet- 

 ric pressure are called isobars or isobaric lines. These are generally 

 drawn for every tenth of an inch, and the distance between two 

 successive lines gives us the gradient. By this we mean the slope 

 of the atmosphere, just as a railway engineer uses the term for the 

 slope of the ground. An engineer will speak of a gradient of 4 in 

 100, meaning a rise of 4 feet in 100 feet of distance ; in weather- 

 reports the units of distance is a degree of 60 geographical miles, 

 and the vertical scale is in hundredths of an inch of the barometer. 

 Hence a gradient of 4 means a rise of 4/iooths ( = i/25th) of an 

 inch in a degree. — CasseWs New Popular Educator for Dec. 



