The United States4 gold coins in circulation now are the double eagle, the eagle, half-eagle, and quarter-eagle, containing, respectively, $20, $10, $5, and $2 1/2. The gold dollar, though no longer coined, was made the standard unit of value in 1900, and since 1837 has contained 25.8 grains .9 fine, or 23.22 grains pure gold. An ounce of standard gold, therefore, will make 18.6 dollar coins, or be worth $18.60, and an ounce of pure gold will make 20.67 coins, or be worth $20.67. The government has minted other gold coins than the above mentioned, but they are not in general circulation and command a premium as souvenirs. They are as follows: the $3 gold piece, coined by act of Congress in 1853 and discontinued in 1890, the weight of which was 74.4 grains; the $1 gold piece, coined first in 1849 and discontinued in 1890, which was of two sizes, large and small, but with the same gold content; the three issues of souvenir gold dollars, the first in 1902 to commemorate the Louisiana Purchase Exposition held at St. Louis, in which issue two different coins were minted, one with a Jefferson head and the other with a McKinley head; the second in 1904 for the Lewis and Clark Exposition held at Portland, Ore.; and the third in 1915 for the Panama-Pacific International Exposition. Only a limited number of these special coins were minted and they were sold at a premium for the benefit of their respective expositions. In 1916 special $1 gold pieces were issued for the McKinley Memorial.