Yes, the coin is 90% silver.
Barber quarters are made of 90% silver and 10% copper. The silver weight would be 5.625 grams.
As issued, a 1904 U.S. Barber quarter has a diameter of 24.3mm and is 1.75mm thick.
A US quarter dated 1915 does contain 90% silver, so yes it is silver
Kennedy half, Walking Liberty half, Barber half, Barber quarter, Washington quarter, Silver Eagle and many others
Your coin is normally referred to as a "Barber" quarter after its designer, Charles Barber. There's more information at the Related Question link.
Yes, it contains 5.625 grams out silver.
All Barber half dollars weigh 12.5 grams (slightly less if heavily worn) and contain 90% silver with 10% copper.
Silver Eagle, Gold Eagle, Kennedy Half Dollar, Walking Liberty Half Dollar, Washington Quarter, Standing Liberty Quarter, Barber Half Dollar, Barber Quarter Dollar and others from the 18th and 19th centuries.
Pre-1965 silver quarters are 90% silver with a total weight of 6.25 grams, which is then 5.625 grams of silver per quarter.
A Barber Quarter weighs 6.25 grams and is 90% silver. This equates to Actual Silver Weight (or "ASW") of 0.1818 troy ounces. As of December 5, 2008 (with silver at $9.48/troy ounce), that is a "melt value" of about $1.72. Depending on condition, however, the numismatic value (that is, the value to a collector) is likely quite a bit higher.
No state quarters minted in either Philadelphia or Denver contain silver.
In 1913, the US mint was producing Barber dimes. They were named after their designer Charles Barber. They contain 90% silver and 10% copper.