SCIENTIFIC TERMS. 143 



Heat, the sensation caused by the action of fire. 



Helioscope, a peculiar kind of telescope. 



Hydrates, those substances which have such an affinity for water as to 

 solidify it Slaked lime is a hydrate of lime. 



Hydraulics, that science which teaches the velocity and force of fluids 

 in motion. The construction of water engines of all kinds depends 

 on Hydraulics. See page 47. 



Hydrogen, one of the constituents of water. See page 75. 



Hydrometer, an instrument to show the strength of spirits. 



Hydrostatic-Balance, an instrument for ascertaining the specific gravity 

 of bodies. See page 46. 



Hydrostatics, that science which teaches the weight and pressure of 

 fluids. See page 45. 



Hygrometer, an instrument for measuring the moisture of the air. 



Jargon, a precious stone found in Ceylon. 



Ice, the state of water deprived of its caloric. 



Ignis-fatuus, a meteor occasionally seen in marshy places. See page 57. 



Iodide, a compound formed by the union of Iodine with any substance. 

 For Iodine, see page 21. 



Iridium, a newly discovered metal, so named from Iris, the rainbow, 

 in consequence of the different colours its salts assume. It is not 

 much used. 



Iron, a well-known metal, exceedingly malleable, ductile, and elastic ; 

 it is easily oxidized, and will burn in oxygen gas. It combines with 

 most of the acids, as well as forms other chemical combinations, 

 which are used in painting, dyeing, and medicine. That substance 

 known by the name of black lead is a Carburet of iron ; Bed Ochre 

 is an ore of iron ; Umber is a compound of the ores of iron and 

 manganese ; and Emery is an oxide of iron. 



Kaleidoscope, an optical instrument formed by a combination of mirrors 

 so as to produce a symmetrical reflection of different objects. 



Kelp, a kind of soda obtained from sea-weed dried and burnt. It is 

 used in making certain kinds of glass. 



Lac, a well-known substance used for making sealing-wax. It is the 

 production of an insect 



Lapis Lazuli, a stone of a beautiful blue colour, very*>rettily spotted 

 with gold-coloured spangles. Painters form their ultramarine from 

 Lapis Lazuli calcined. 



Lead, a well-known metal, particularly abundant in Great Britain. The 

 lead-mines of Derbyshire alone yield 15,000 tons per annum. Lead 

 possesses but very little tenacity and ductility, and with the exception 

 of platinum, gold, and mercury, it is the heaviest of the metals. It 

 has a great affinity for oxygen, and will form various chemical com- 

 binations. Its oxides are used in dyeing, calico printing, making 

 glass, earthenware, &c. 



Lens, a piece of glass or other transparent body whose sides are either 

 convex or concave, used for the purpose of converging or diverging 

 the rays of light. 



Lever, a bar of iron or wood, used as one of the mechanical powers. 

 See page 67. 



