Editing Latent heat (section)

=== Latent heat ===
[[File:Black_Joseph_(cropped).jpg|thumb|196x196px|Joseph Black]]
The English word ''[[wikt:latent#English|latent]]'' comes from Latin ''[[wikt:latens#Latin|latēns]]'', meaning ''lying hidden''.<ref>{{OEtymD|latent}}</ref><ref>Lewis, Charlton T. (1890). ''An Elementary Latin Dictionary''. Entry for [https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0060%3Aentry%3Dlatens latens].</ref> The term ''latent heat'' was introduced into calorimetry around 1750 by [[Joseph Black]], commissioned by producers of [[Scotch whisky]] in search of ideal quantities of fuel and water for their distilling process to study system changes, such as of volume and pressure, when the thermodynamic system was held at constant temperature in a thermal bath. 

It was known that when the air temperature rises above freezing—air then becoming the obvious heat source—snow melts very slowly and the temperature of the melted snow is close to its freezing point.<ref name=":0" /> In 1757, Black started to investigate if heat, therefore, was required for the melting of a solid, independent of any rise in temperature. As far Black knew, the general view at that time was that melting was inevitably accompanied by a small increase in temperature, and that no additional heat was needed beyond what this increase in temperature would require in itself. Soon, however, Black was able to show that much more heat was required during melting than could be explained by the increase in temperature alone.<ref>{{Cite book |last=Ramsay |first=William |url=https://books.google.com/books?id=micIAAAAIAAJ&pg=PA44 |title=The Life and letters of Joseph Black, M.D. |date=1918 |publisher=Constable |pages=44 |language=en}}</ref><ref>{{Cite book |last=Black |first=Joseph |author-link=Joseph Black |url=https://books.google.com/books?id=lqI9AQAAMAAJ&pg=PA111 |title=Lectures on the Elements of Chemistry: Delivered in the University of Edinburgh |date=1807 |publisher=Mathew Carey |editor-last=Robinson |editor-first=John |edition= |pages=111–112 |language=en}}</ref> He was also able to show that heat is released by a liquid during its freezing; again, much more than could be explained by the decrease of its temperature alone.<ref>{{Cite book |last=Black |first=Joseph |url=https://books.google.com/books?id=lqI9AQAAMAAJ&pg=PA112 |title=Lectures on the Elements of Chemistry: Delivered in the University of Edinburgh |date=1807 |publisher=Mathew Carey |editor-last=Robison |editor-first=John |pages=112 |language=en}}</ref>

Black would compare the change in temperature of two identical quantities of water, heated by identical means, one of which was, say, melted from ice, whereas the other was heated from merely cold liquid state. By comparing the resulting temperatures, he could conclude that, for instance, the temperature of the sample melted from ice was 140&nbsp;°F lower than the other sample, thus melting the ice absorbed 140 "degrees of heat" that could not be measured by the thermometer, yet needed to be supplied, thus it was "latent" (hidden). Black also deduced that as much latent heat as was supplied into boiling the distillate (thus giving the quantity of fuel needed) also had to be absorbed to condense it again (thus giving the cooling water required).<ref>{{Cite episode |title=Credit Where It's Due |series=The Day the Universe Changed |series-link=The Day the Universe Changed |network=BBC |date=1979 |number=6 |time=50 (34 minutes) |credits=[[James Burke (science historian)|Burke, James]]}}</ref>

==== Quantifying latent heat ====
In 1762, Black announced the following research and results to a society of professors at the University of Glasgow.<ref name=":2">{{Cite book |last=Black |first=Joseph |author-link=Joseph Black |url=https://books.google.com/books?id=lqI9AQAAMAAJ&pg=PA120 |title=Lectures on the Elements of Chemistry: Delivered in the University of Edinburgh |date=1807 |publisher=Mathew Carey |editor-last=Robinson |editor-first=John |edition= |pages=120 |language=en}}</ref> Black had placed equal masses of ice at 32&nbsp;°F (0&nbsp;°C) and water at 33&nbsp;°F (0.6&nbsp;°C) respectively in two identical, well separated containers. The water and the ice were both evenly heated to 40&nbsp;°F by the air in the room, which was at a constant 47&nbsp;°F (8&nbsp;°C). The water had therefore received 40&nbsp;–&nbsp;33&nbsp;=&nbsp;7 “degrees of heat”. The ice had been heated for 21 times longer and had therefore received 7&nbsp;×&nbsp;21&nbsp;=&nbsp;147 “degrees of heat”.{{refn|group=lower-alpha|These “degrees of heat” were context-dependent and could only be used when circumstances were identical—except for the one differing factor to be investigated. When Black investigated specific heat, the “degrees of heat” were based on change in temperature multiplied by mass. When Black investigated latent heat, they were based on change in temperature multiplied by time passed. Clearly these units were not equivalent.}} The temperature of the ice had increased by 8&nbsp;°F. The ice had thus absorbed 8 “degrees of heat”, which Black called ''sensible heat'', manifest as a temperature increase, which could be felt and measured. In addition to that, 147 – 8 = 139 “degrees of heat” were absorbed as ''latent heat'', manifest as phase change rather than as temperature change.<ref name=":12">{{Cite book |last=Ramsay |first=W. |author-link=William Ramsay |url=https://books.google.com/books?id=micIAAAAIAAJ&pg=PA44 |title=The life and letters of Joseph Black, M.D. |publisher=Constable |year=1918 |pages=44}}</ref><ref>{{Cite book |last=Black |first=Joseph |author-link=Joseph Black |url=https://books.google.com/books?id=lqI9AQAAMAAJ&pg=PA115 |title=Lectures on the Elements of Chemistry: Delivered in the University of Edinburgh |date=1807 |publisher=Mathew Carey |editor-last=Robinson |editor-first=John |edition= |pages=115–117 |language=en}}</ref> 

Black next showed that a water temperature of 176&nbsp;°F was needed to melt an equal mass of ice until it was all 32&nbsp;°F. So now 176&nbsp;– 32&nbsp;=&nbsp;144 “degrees of heat” seemed to be needed to melt the ice. The modern value for the heat of fusion of ice would be 143 “degrees of heat” on the same scale (79.5 “degrees of heat Celsius”).<ref name=":13">{{Cite book |last=Ramsay |first=W. |author-link=William Ramsay |url=https://books.google.com/books?id=micIAAAAIAAJ&pg=PA45 |title=The life and letters of Joseph Black, M.D. |publisher=Constable |year=1918 |pages=45}}</ref><ref name=":2" /> 

Finally, Black increased the temperature of a mass of water, then vaporized an equal mass of water by even heating. He showed that 830 “degrees of heat” was needed for the vaporization; again based on the time required. The modern value for the heat of vaporization of water would be 967 “degrees of heat” on the same scale.<ref name=":14">{{Cite book |last=Ramsay |first=W. |author-link=William Ramsay |url=https://books.google.com/books?id=micIAAAAIAAJ&pg=PA45 |title=The life and letters of Joseph Black, M.D. |publisher=Constable |year=1918 |pages=45–46}}</ref>