Why didn't Herschel discover ultraviolet heat with a thermometer like infrared?

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I was researching about the discovery of electromagnetism and how Johann Ritter (Discoverer of ultraviolet light) used silver chloride to discover the heat of ultraviolet light but why couldn't Sir william Herschel discover it with just a thermometer like infrared light?
 
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The sun emits much more infrared than ultraviolet. In addition, UV does not pass through most glasses, probably including the prisms used to separate infrared from visible light.
 
The turning black of silver chloride by light is due to the absorption of UV light by the molecule with its subsequent dissociation into Silver and Chlorine. Heat played no part in Ritter's discovery.