{|class="tablecolhdborder floatleft"

|-style="text-align: center; border: 1px solid darkgray; border-style: none solid solid solid" |colspan=2|Substance. ||Observer. ||Date. |- |colspan=2 style="text-align: center;"|Platinum || || |- |colspan=2|Chloride in ether||Gehieri || 1804 |- |colspan=2|Chloride with lime||Herschel || 1840 |- |colspan=2|Iodide ||Herschel||1840 |- |Bromide||rowspan="2" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}}$||rowspan="2"|Hunt ||rowspan="2"|1844 |- |Cyanide |- |colspan=2|Double cloride platimum and potassium||Doberiner||1828 |- |colspan=2|style="text-align: center;"|Mercury || || |- |colspan=2|Oxide (mercurous) ||Gay-Lusac and Thénard||1811 |- |colspan=2|Oxide ||Davy ||1812 |- |colspan=2|Oxide (mercuric) ||Davy||1797 |- |rowspan="2"|Oxide (more accurate observations)||rowspan="2" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}}$||Abidguard||1797 |- |Harup not till ||1801 |- |colspan=2|Chloride (mercurous)||K. Neumann previous to ||1739 |- |colspan=2|Chloride (mercuric) ||Boullay ||1803 |- |colspan=2|Chloride with oxlic acid ||Bergmann ||1776 |- |colspan=2|Sulphate ||Myer ||1764 |- |colspan=2|Oxlate (meruric)||Bergman||1776 |- |colspan=2|Oxlate (mercurous)||Harff||1836 |- |colspan=2|Sulphate and ammonia (mercurous)||Fourcroy||1791 |- |colspan=2|Acetete (mercurous) ||Garot||1826 |- |colspan=2|Bromide (mercuric)|||Lowig||1826 |- |rowspan="2"|Iodide (mercurous)||rowspan="2" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}}$||Torosewicz||1836 |- |Artus||1836 |- |colspan=2|Iodide (mercuric)||Field||1836 |- |colspan=2|Citrate (mercuric)||Harf||1836 |- |colspan=2|Tartrate and potassium (mercurous)||Carbonell and Bravo||1831 |- |colspan=2|Carbonate (mercuric)||Davy||1812 |- |colspan=2|Nitrate||Herschel ||1840 |- |colspan=2|Sulphide (mercuric)||Viruvius ||1b.c. |- |colspan=2|style="text-align: center;"|Iron|| || |- |colspan=2|Sulphate (ferrous) ||Chastaing ||1877 |- |colspan=2|Chloride (ferric) and alchol ||Bestuscheff ||1725 |- |colspan=2|Chloride and ether ||Klaproth ||1725 |- |colspan=2|Oxalate (ferric) ||Dobereiner ||1831 |- |colspan=2|Ferrocyanide of potassium ||Henrich ||1808 |- |colspan=2|Sulphocyanide ||Grotthus ||1818 |- |colspan=2|Prussian blue ||Scopoli ||1783 |- |colspan=2|Ferric citrate with ammonium ||Herschel ||1840 |- |colspan=2|Ferric tartrate ||Herschel ||1840 |- |colspan=2|Chromate ||Hunt ||1844 |- |colspan=2|style="text-align: center;"|Copper.|| || |- |colspan=2|Chloride (cupric dissolved in ether)||Gehlen ||1804 |- |colspan=2|Oxalate with sodium||A. Vogel ||1813 |- |Chromate||rowspan="5" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right\}\,}}$||rowspan="5"|Hunt ||rowspan="5"|1844 |- |Chromate with ammonium |- |Carbonate |- |Iodide |- |Sulphate |- |colspan=2|Chloride (cuprous)||A. Vogel ||1859 |- |rowspan="2"|Copper plates (iodized)||rowspan="2" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}}$||Kratoch ||1841 |- |Talbot||1841 |- |colspan=2 style="text-align: center;"|Maganese.|| || |- |colspan=2|Sulphate|| Brandenburg||1815 |- |colspan=2|Oxalate||Suckow ||1813 |- |colspan=2|Potassium permanganate||Frommberg ||1824 |- |colspan=2|Peroxide and cyanide of potassium||Hunt||1844 |- |colspan=2|Chloride||Hunt||1844 |- |colspan=2 vstyle="text-align: center;"|Lead. || || |- |colspan=2|Oxide||Davy ||1802 |- |Iodide||rowspan="2" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}}$||rowspan="2"|Schonbein ||rowspan="2"|1850 |- |Sulphite |- |colspan=2|Peroxide||Gay-Lussac||1811 |- |colspan=2|Red lead and cyanide of potassium||Hunt||1844 |- |colspan=2|Acetate||Hunt||1844 |- |colspan=2 style="text-align: center;"|Nickel. || || |- |Nitrate||rowspan="3" style="text-align: right; border-style: none none none none"|${\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}}$||rowspan="3"|Hunt ||rowspan="3"|1844 |- |Nitrate with ferro-prussiates |- |Iodide |- |colspan=2 style="text-align: center;"|Tin. || || |- |colspan=2|Purple of cassius||Uncertain || |}

Substance.		Observer.	Date.
Various Substances.
Cobalt salts		Hunt	1844
Arsenic sulphide (realgar)		Sage	1803
Antimony Sulphide		Suckow	1832
Bismuth salts	${\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}}$	Hunt	1844
Cadmium salts
Rhodium salts
Vanadic salts		Roscoe	1874
Iriduim ammoniaum chloride		Dobereiner	1831
Potassium bichromate		Mungo Ponton	1838
Potassium with iodide of starch		Becquerel	1840
Metallic chromates		Hunt	1843
Chlorine and hydrogen		Gay-Lussac and Thénard	1809
Chlorine (tithonized)		Draper	1842
Chlorine and ether		Calhours	1810
Chlorine in water		Bertollet	1785
Chlorine and ethylene		Gay-Lussac and Thénard	1809
Chlorine and canon-monoxide		Davy	1812
Chlorine and Hydrocyanic acid		Henry	1821
Bromide and hydrogen		Balard	1832
Iodine and ethylene		Faraday	1821
Cyanogen, solution of		Pelouze and Richardson	1837
Various other Methyl compounds		Cahours	1846
Hydrocyanic acid		Totosewicz	1836
Hypochlorites (calcium and potassium)		Dobereiner	1813
Uranium chloride and ether		Gehlen	1804
Molybdenite of potassium and tin salts		Jager	1800
Crystallization and slats under influence of light	${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}}$	Cahours	1846
		Chapptal	1788
		Dize	1789
Phosphorous (in hydrogen, nitrogen &c.)		Brockmann	1800
Phosphuretted hydrogen		A. Volel	1812
Natric acid		Scheele	1777
Hog's fat		Vogle	1806
Palm oil		Fier	1832
Asphalt		Niepce	1814
Resins (mastic, sandarac, gamboge, ammoniacum, &c.)		Senebier	1782
Guaiacum		Hagemann	1782
Bitumens all decomposed, al residues of essential oils		Daguerre	1839
Coloured extracts from flowers		Senebier	1782
Similar colouring matters spread upon paper		Herschel	1842
Yellow wax bleached		Pliny	1st cent.a.d.
Eudoxia macrembolitissa (purple dye)			10th cent.
Other purple dyes	${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}}$	Cole	1684
		Réaumur	1711
Oils generally		Senebier	1782
Nitric ether		Senebier	1782
Nioctine		Henry & Boutron-Charlard	1836
Santonine		Merk	1888

Effect of Hydrogen Peroxide on Sensitive Plates.—Dr W. J. Russell made a series of experiments on the effect of exposure of sensitive plates to the action of vapours and gases for long periods. It has long been known that contact of plates with such substances as wood caused a sensitive surface to so "fog" on development. By a somewhat exhaustive series of experiments, Russell showed that the probable cause of this fog is hydrogen peroxide, since substances which favoured its formation produced the same effect. This is somewhat remarkable, as this same substance will completely destroy the effect that light has had on a sens1t1ve plate; indeed, it affords one way of destroying a light image on a sensitive collodion plate. The experiment of Russell give a warning to store exposed plates for brief periods. It appears that negatives wrapped in paraffin paper are secure from this danger.

The Application of Photography to Quantitative Measures.—In order to employ photography for the measurement of light it was necessary that some means should be devised by which the opacity of the deposit produced on the development of a plate could be determined. It is believed that in 1874 the first attempt was made by Sir W. Abney to do this. In the Phil. Mag. he

showed how density could be measured by means of an instrument, the diaphanometer, he had devised, in which transparent