222 APPENDIX. [No. XIV. 



especially in the former case, cause irremediable injury. Some time since, 

 I had a case under my care, where a small portion of a needle was 

 introduced into one of the joints of the finger, but of which no indication 

 existed beyond the effects which might have been expected from the 

 presence of a foreign body. The exact spot of its insertion was unknown ; 

 and indeed it was equally uncertain whether it was inserted or not. 

 Subsequently the joint swelled, suppurated, and discharged, and a small 

 piece of needle was found firmly impacted in the bone. Now, a very small 

 piece of foreign matter is capable of producing these disastrous results ; 

 and, on having weighed the piece discharged in this case, I found that it 

 scarcely amounted to the | of a grain. To this case I shall again draw your 

 attention, when I come to my lectures on the diseases of the joints, because 

 it showed accidentally, on the human subject, the course of the inflam- 

 mation and suppuration of the part, the subsequent ulceration of the 

 cartilages and osseous tissue, and, finally, the course of the reparative 

 process, by the termination of the inflammation by anchylosis. On 

 reference to my note -book, for the purpose of studying this case, it 

 occurred to my mind that, had I known that the needle was actually 

 present, and could have demonstrated its exact spot, I might possibly 

 have averted the present inconvenience of a stiff joint to the unfortunate 

 sufferer ; and, after having carefully considered the matter, a plan 

 suggested itself to my mind for the detection of needles in future cases. 

 You are all acquainted with the curious condition which steel assumes 

 under certain circumstances, whereby it evinces properties which are 

 called magnetic ; you know, moreover, that like magnetic poles repel, and 

 opposite attract each other. You have, therefore, but to render a piece 

 of enclosed steel a magnet, and you will be able not only to ascertain 

 its presence, but to determine by its polarity its general direction; 

 and, by the amount of magnetism it evinces, you may even infer its pro- 

 bable bulk. 



When you suspect the presence of a piece of needle, or other steel 

 instrument, you must subject the suspected part to a treatment calculated 

 to render the needle magnetic ; and there are two principal methods by 

 which this object may be effected : the first, by transmitting a galvanic 

 current, at right angles, to the suspected part ; the second, by placing a 

 large magnet near the part affected, so that the object may be magnetized 

 by induction. You may accomplish the first end by taking a copper wire, 

 covered with cotton, or still better with silk (in fact, you may employ the 

 covered wire as generally used for the formation of electro-magnets), and 

 wind it round the parts suspected to contain 

 steel, several times, so that the same current 

 may act at right angles, many times, upon 



^^ , ^ the piece of steel; you may then take a 



V V galvanic battery (one of my little tumbler 



FIG. 11. Covered Wire, as gene- batteries will amply suffice), and connect one 

 rally used for the formation of en( j o f the wire to the zinc, the other to the 



Electro-Magnets. ij.--j-i rm j- L n -11 



platinized silver. The adjoining cut, ng. 11, 



shows the general arrangement which may be adopted to effect this 

 object. The current might be continued for half an hour, or more, when 

 the steel would become magnetized, and thereby give strong indications 

 of its presence. 



