320 EXPLOSIVE AGENTS 



tube, nt the centre, and a board was fixed on a post independent of the tube, and at right 

 angles to it ; the pencil was pressed against the board by a spring, and the rise ;m< I tall, 

 and the lateral motions of the tube, were consequently traced on the board. In this 

 way a very interesting diagram was taken daily. The lowest part of each figure is the 

 starting point, or normal position of the tube, to which the pencil always accurately 

 returns during the night. As soon as the sun rises in the morning it starts towards 

 the right hand, rising obliquely, the top and one side of the tube being warmed, and 

 the bottom and opposite side remaining unaffected. It continues thus till one o'clock, 

 when the sun, having ceased to shine on the southern side, begins to warm the 

 northern side, the top still retaining its high temperature ; the tube thus acquires a 

 nearly horizontal motion towards the left hand, the slight descent in the line indicating 

 the diminishing effects of the sun on the top as it gradually sinks. The greatest deflec- 

 tion to the left hand is not attained until sunset, after which the tube rapidly descends 

 in a uniformly-curved line to its resting point. In the summer time this point is hardly 

 attained before the rising sun compels it to commence its journey anew. When the 

 sun is frequently obscured by passing clouds, very curious diagrams are obtained. 

 During the absence of the sun the tube begins to cool rapidly, and to return to its 

 normal position ; every passing cloud is thus beautifully recorded. 



The middle of the centre arch of Southwark Iron Bridge rises one inch in the 

 height of summer. When great lengths of iron pipe are laid down for the convey- 

 ance of steam or hot water, sliding joints are necessary to prevent destruction either 

 of the apparatus or of the building in which it is placed. 



The practical applications made of the expansion and contraction of metals by heat 

 are many. The tire of a wheel is put on hot, and by its contraction on cooling, 

 firmly binds the other parts of the wheel together; boiler-plates are riveted with 

 red-hot rivets ; collars of metal are driven on while hot, and the like. 



Mollard drew together the walls of a building that had bulged, by screwing up bars 

 of iron tight to the walls while they were hot, and a similar process was adopted in the 

 Cathedral of Armagh. 



Playfair and Joule (' Chemical Society's Memoirs ') have made a valuable series of 

 researches on the expansion of bodies by heat, principally salts ; these have not, how- 

 ever, any sufficient practical bearing to occupy our space. 



EXPLOSIVE AGENTS. The peculiar characters of such explosive compounds 

 as GUNPOWDER, GUN-COTTON, DYNAMITE, NITRO-GLYCERINE, and the like, will be 

 found under those articles respectively. In this place it is intended to describe in a 

 general way some explosive compounds which are less known, and which have not as 

 yet been employed successfully. 



Chi or ate -of -Potash Compounds. The peculiar powers of chlorate of potash, and its 

 violent oxidising properties have led to many experiments. This salt has been mixed 

 with the prussiates of potash, with sugar and with starch, and sometimes with the 

 addition of sulphur. These gunpowder substitutes have been known as white gun- 

 powder, German gunpowder, &c. Ingenious attempts have been made to reduce the 

 dangerous nature of the chlorate-of-potash mixtures by combining them with inert 

 materials. One kind consists of mixtures of the salt with organic substances con- 

 taining, in addition to carbon, a considerable proportion of hydrogen, such as pow- 

 dered nut-galls, tannin, and resins. Horsley's powder was somewhat of this character, 

 with the addition of some nitro-gtycerine. That of Messrs. Hochstader, which was 

 introduced in 1860, and subsequently modified by M. Reichen, was somewhat similar. 

 Strips of blotting-paper were soaked in a pasty mass consisting essentially of a mix- 

 ture of chlorate of potash, saltpetre, charcoal (and small quantities of other readily 

 oxidisablb substances), together with a little gum or other binding material dissolved 

 in water. The paper becomes coated with the explosive mixture, and at the same 

 time impregnated with the oxidising salts, of which it absorbs a part of the solution ; 

 the strips are rolled up tightly while wet, and when dry they become hard and com- 

 pact cylindrical masses, which are violently explosive when confined, but resist detona- 

 tion when submitted to percussion or friction. One of these white gunpowder* \\as 

 recommended for its safety ; the chlorate of potash and the other ingredient being kept 

 separately until required for use. 



Tutonite is a blasting agent of this class. It has the peculiarity of being made 

 up in the form of hard pellets or disks, instead of being in a granulated form or in 

 powder. 



Safety Powder was a blasting agent devised by Mr. Kollow, and manufactn: 

 the works at South Down, opposite to Devonport. It consisted of spent tan and saw- 

 dust, saturated with saltpetre or nitrate of soda, and a little chlorate of potash ; the 

 compound being subsequently crudely mixed with sulphur. (The manufacture \v. - 

 abandoned, the works having been destroyed by fire.) 



Pyrolithc is the name of a blasting powder very nearly resembling that of Mr. 

 Kellow's. 



